Perspective Chapter: Emerging Infectious Diseases as a Public Health Problem

Abstract

Emerging infectious diseases (EIDs) can be defined as diseases that have recently appeared in a population or are rapidly increasing in incidence or geographic range. An “emerging infection” refers to either a new infection that has never appeared before or a known infection that has experienced a recent increase in prevalence. The Human Immunodeficiency Virus (HIV) pandemic and Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) outbreaks are prototypical examples of emerging infectious diseases that were not prevalent before the 1980s and 2003, respectively. On the other hand, a “re-emerging infection” is a familiar infection that resurfaces. The influenza A virus pandemics of 1918, 1957, and 1968 serve as prototypical examples of re-emerging infections. This chapter aims to define the concepts of emerging and re-emerging infectious diseases and explore their main causes, the microorganisms involved, and why they can become significant global public health problems.

1. Introduction

Emerging infectious diseases (EIDs) had been a public health problem for decades and continue to pose a challenge to global health. Throughout history, there have been numerous infectious diseases that were once considered emerging and had a significant impact on human health and society as a whole. The frequency of EIDs has notably increased in the past 30 years, and it is expected to further rise in the near future.

Some examples of historically emerging infections include the 1918 Spanish flu pandemic that claimed millions of lives worldwide [1]. Another example is the emergence of Human Immunodeficiency Virus/Acquired Immunodeficiency Syndrome (HIV/AIDS) in the 1980s, which has caused millions of deaths since its onset and remains a significant public health issue today [2]. In the past decade, we have witnessed the emergence of new infectious diseases, such as the Ebola virus in West Africa in 2014, the Zika virus in South America in 2015, and the Coronavirus 2019 (COVID-19) pandemic, worldwide in 2019.

Emerging infectious diseases can be broadly defined as infections that have recently appeared in a population or have existed but have experienced an increase in incidence or geographic range [3]. They can manifest as rapidly spreading diseases that pose a significant threat to public health and the global economy.

The difference between an emerging infection and a re-emerging infection lies in the temporal and epidemiological context in which they occur. Emerging infections are those that appear for the first time in a population or experience a significant increase in incidence. On the other hand, re-emerging infections are those that have existed previously but have resurfaced or experienced an increase in incidence after a period of decline or apparent control.

Emerging infections are often associated with various factors, including the emergence of new pathogens through genetic mutation, pathogen transfer from one species to another, or the introduction of a pathogen into a susceptible population. Additionally, they can result from diverse and complex factors such as environmental and climatic changes, ecosystem disruptions, human migration, globalization, and intensive agricultural practices [4].

On the other hand, re-emerging infections are diseases that have been present in a population in the past but have experienced a resurgence in their incidence. This can be due to various factors, including the weakening of control measures, the emergence of drug-resistant strains, a decrease in population immunity, or changes in human behavior that increase exposure to the pathogen [5].

In summary, the definition of EIDs can include various categories: [6].

1. True new diseases that arise in humans from zoonotic sources.

2. Previously identified diseases that may have been unknown or prevalent but are now being evaluated.

3. Widely known re-emerging diseases that increase in frequency and may emerge as coinfections that facilitate their spread (such as HIV).

Over 60% of these new EIDs are associated with zoonoses, where 23% are vector-borne diseases (VBDs) and 72% originate from wildlife [7]. EIDs can arise from different sources, such as changes in the environment, pathogen evolution, drug resistance, and globalization. Early identification and rapid response are crucial in preventing the spread of emerging diseases.

The factors associated with the emergence of diseases include a variety of biological, environmental, social, and cultural elements. Some of these factors include:

• Climate change and destruction of natural habitats, along with changes and geographic expansion of vector habitats.

• Globalization and increased international trade and travel.

• Increased population density and urbanization.

• Changes in agriculture and animal farming.

• Drug resistance and antimicrobial therapy.

• Evolution and mutation of pathogens.

• Emergence of new technologies and their impact on public health.

• International travel.

These factors can interact and often unpredictable ways, leading to the emergence of new infectious diseases.

Emerging infectious diseases have historically represented the most feared plagues for humans, and new infections continue to arise today, giving rise to new significant epidemics and pandemics. In fact, the emergence of the new coronavirus “COVID-19” in December 2019 is a clear example of what these emerging diseases represent, and we will delve further into this chapter. Let us examine some of the diseases that, due to their characteristics, have great potential to become a global problem in the short or medium term.

First, the influenza virus, which has caused historic pandemics throughout the centuries. In the last 100 years, there have been four influenza pandemics: H₁N₁ (Spanish flu) in 1918 with 50 million deaths, H₂N₂ (Asian flu) in 1957, H₃N₂ (Hong Kong flu) in 1968, and the new H₁N₁ (swine flu) in 2009. Influenza type A virus has the highest potentiality to become a new pandemic in the future due to its ability to undergo significant genetic modifications through mutations and recombination, resulting in the emergence of new viral variants with novel characteristics. One crucial aspect of influenza type A virus that makes it more concerning is its capacity to infect both humans and animals, including aquatic birds and mammals, known as zoonotic hosts. This creates an environment conducive to interspecies transmission and the possibility of genetic recombination between human and animal strains of the virus. A specific subtype of influenza A virus that has been a cause for concern in recent decades is H₅N₁, known as highly pathogenic avian influenza. This subtype has been responsible for outbreaks in poultry and wild birds and has occasionally transmitted the virus to humans, resulting in severe infections and mortality, though its mutations have not yet been allowed for sustained human-to-human transmission. Other subtypes, such as H₇N₉ and H₉N₂, have also shown the ability to infect humans and have been under surveillance due to their pandemic potential.

Second, the Ebola virus can cause hemorrhagic fever. It is essential to highlight that the Ebola virus has certain characteristics that give it a high potential to become a new global health problem. Although the outbreaks that have occurred so far have been regional, it is crucial to consider some factors that make this virus a concern. The Ebola virus is highly lethal, with mortality rates reaching up to 90%. This characteristic is worrying since, in the event of broader transmission, it could cause a significant number of deaths and overwhelm healthcare systems. The virus is a zoonosis originating in fruit bats and occasionally transmitted to humans through contact with wild animals. The major concern would be if the virus jumps to new species and adopts a more efficient form of human-to-human transmission. Transmission from an infected person to a healthy one occurs through direct contact with infected blood and fluids. If a variant with higher human-to-human transmission capability were to appear, it could trigger a faster and broader spread.

Finally, the dengue virus is a mosquito-borne disease endemic in more than 100 countries, mainly in tropical and subtropical regions. In recent years, outbreaks have been reported in various European countries, indicating a potential spread beyond its usual territories. The widespread distribution of the Aedes aegypti mosquito, the main vector of dengue, coupled with increased trade, international travel, and climate change—which can influence the distribution and activity of mosquito vectors, expanding their geographic range—increase the likelihood of its introduction to new areas, potentially triggering outbreaks and rapid global spread.

There are four distinct serotypes of the dengue virus (DEN-1, DEN-2, DEN-3, and DEN-4), and a previous infection with one serotype provides lifelong immunity only against that specific serotype. However, subsequent infection with another serotype increases the risk of developing severe dengue, a more severe form of the disease. Thus, the coexistence of multiple serotypes in a region increases the possibility of outbreaks and clinical complications. Although there are dengue vaccines available, a universal vaccine providing complete protection against all serotypes has not been developed yet, making control and prevention more challenging.

In this chapter, we will analyze the current state of knowledge about emerging infectious diseases, describe some characteristics of the microorganisms causing these diseases, and detail the factors contributing to their emergence. We will also review their global impact on public health.

2. Factors associated with emerging infectious diseases

Although, as we have seen previously, there are various factors associated with the increase in EIDs, we can summarize them as: social and behavioral changes, environmental changes, and changes in the causative microorganisms. These factors would act synergistically, increasing the emergence of certain pathogens, affecting transmission mechanisms, and leading to greater host susceptibility [8].

Table 1 provides a more detailed classification of the factors that, either individually or in combination, enhance the occurrence of EIDs.

Environmental factors	Anthropological factors	Pathogenetic balance	Virulence
Temperature; Humidity; Drought	Socioeconomic factors; Globalization; Land use	Host immunity; Geographical redistribution of the pathogen; Exposure to a new species; Recombination and adaptation	Increased cytotoxicity; Increased susceptibility; Antimicrobial resistance; Emergence of a pathogen in a new host

Table 1.

Factors that enhance the appearance of emerging infectious diseases (EIDs).

2.1 Demographic factors

Demographic factors play a significant role in the emergence and spread of emerging infectious diseases. Factors, such as rapid population growth, uncontrolled urbanization, and changes in population distribution, are key elements that affect the dynamics of infectious diseases. These demographic factors interact with other socioeconomic and environmental aspects to influence the emergence and spread of diseases.

First, rapid population growth can increase population density in urban areas, facilitating disease transmission. A study by Smith et al. [9] pointed out that urban growth creates conducive conditions for the spread of infectious diseases, as proximity among individuals facilitates transmission.

Second, changes in population distribution can also play a role in EIDs. Human mobility, including international travel, can lead to the rapid global spread of diseases. In 2019, 1.5 billion tourists traveled outside their borders worldwide, facilitating the spread of these infectious diseases within days or weeks, as demonstrated by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) pandemic. Tatem et al. [10] demonstrated that patterns of human mobility can predict the spread of infectious diseases, such as influenza, across different geographic regions. Demographic changes in population size and density through urbanization can also affect infection dynamics. For example, influenza tends to have more persistent outbreaks in densely populated urban regions, with a similar pattern observed in the early COVID-19 pandemic [11].

If demographic change has significantly altered the context of infectious diseases in recent years, an even greater effect can be attributed to changes in the emergence of immunomodulatory infections, which, in turn, can affect other infections. For instance, the emergence of HIV has amplified the burden of tuberculosis [12].

On the other hand, demographic changes such as population aging also have a significant influence on the emergence of EIDs. Older individuals tend to have a weakened immune system, making them more susceptible to infections. Additionally, the presence of comorbidities in the elderly population can complicate the body’s response to infections.

These demographic factors have a significant impact on the emergence and spread of EIDs. Rapid population growth, uncontrolled urbanization, changes in population distribution, and population aging are key demographic elements that influence the dynamics of infectious diseases. Understanding and addressing these demographic factors is crucial for effectively preventing and controlling emerging infectious diseases.

2.2 Social factors

Transformations in human behavioral patterns, cultural practices, and social interactions can significantly influence the dynamics and spread of diseases. One social factor that can contribute to the emergence of EIDs is the increase in mobility and international travel. Mass movements of people facilitate the rapid spread of pathogens across different geographical regions. Furthermore, the globalization of trade and tourism has increased interconnectivity between communities, allowing for the introduction and dissemination of diseases in new areas [13].

Changes in food production and consumption practices can also have a significant impact. Intensive agriculture and overcrowded animal farming increase the risk of zoonotic disease transmission, facilitating the transmission of diseases from animals to humans. The mass production of food, particularly animal protein, often incorporates the use of antibiotics to enhance the growth and productivity of livestock such as chickens, pigs, and cattle. Additionally, the distribution of food to markets can introduce new pathogens into the food chain through food handlers, even distal to the markets where they are sold.

On the other hand, changes in behaviors and social interactions can also influence the spread of emerging infectious diseases. Risky sexual practices, such as unprotected sex and promiscuity, can increase the transmission of sexually transmitted diseases (STDs) such as HIV/AIDS and syphilis. Likewise, changes in personal hygiene behaviors and healthcare practices can affect the transmission of respiratory, gastrointestinal, and contact diseases.

2.3 Climate change

The negative impact of infectious diseases on health and well-being is intrinsically related, as we have seen before, to a combination of multiple stressful or driving factors, to which we should add poor sanitation, access to clean water and food, the quality of public health services, political instability and conflicts, drug resistance, and movements of animal and/or human populations. Changes in climate patterns, environmental degradation, and ecosystem disruption can trigger the emergence and expansion of infectious diseases, creating a conducive environment for their transmission.

How we shape and adapt to the environment, through our impact on land use (deforestation/reforestation and agricultural activities), the construction of artificial water bodies or dams, and measures taken to control infectious diseases, such as the development of vaccines and drugs, insecticide spraying, distribution of insecticide-treated bed nets, and the development of rapid diagnostic tests, are also critical factors affecting the transmission of infectious diseases [14]. Climate has a direct impact on the dynamics of a subset of infectious diseases, including vector-borne diseases (VBDs), some waterborne diseases such as cholera, and other pathogens transmitted through soil and food. The climate also has multiple indirect effects through socioeconomic factors; for example, floods can hinder ongoing disease control measures, including vector control [15].

Infectious VBDs are primarily transmitted by arthropod vectors, which are particularly sensitive to climate changes for several reasons. Arthropods are ectothermic, with their internal temperature regulated by external environmental conditions [16]. Their larval developmental stage usually requires the presence of water bodies and/or specific humidity conditions. Vector biting rates tend to increase with temperature up to an upper threshold, after which they decrease. The development and replication of vector-transmitted pathogens (the extrinsic incubation period or EIP) or in the environment also occur faster at higher temperatures [17]. Additionally, the development and survival of vectors are significantly affected by seasonal conditions [18].

Many transmission-related life cycle traits of the mosquito (biting rate, adult lifespan, population size, and distribution) and the pathogen (extrinsic incubation rate) are temperature sensitive, and oviposition patterns depend on water availability [19]. Consequently, the geographical range for dengue, malaria, and other vector-borne diseases [20, 21, 22, 23] is affected by the local climate, and there is substantial effort to understand how these ranges may change with climate change [24, 25, 26]. For certain vector-borne diseases such as Zika virus disease, climate change may lead to an expanded range [27]. However, for other diseases, such as malaria, climate change may shift the spatial range of the infection to higher latitudes. As ever, the footprint of human interventions may loom larger than these changes in local conditions.

One of the main effects of climate change is the increase in global temperature. Global warming has led to the expansion of areas conducive to the proliferation of vectors, such as mosquitoes, which transmit diseases such as dengue, malaria, and the Zika virus. A study conducted by Caminade et al. [28] found a significant relationship between temperature increase and the geographical expansion of disease-transmitting vectors.

Furthermore, climate change can alter rainfall patterns and season distribution, impacting the spread of waterborne diseases such as cholera. Heavy rains and floods can contaminate sources of drinking water and facilitate pathogen spread, while prolonged droughts can increase water scarcity and inadequate hygiene conditions. Ecological change, such as deforestation and ecosystem degradation, can also contribute to the emergence of emerging infectious diseases. Destruction of natural habitats and loss of biodiversity can lead to increased contact between humans and disease-carrying animals. This increases the risk of zoonotic disease transmission, such as West Nile virus and Ebola virus [29]. Additionally, changes in migratory patterns of animal species due to climate change can alter disease dynamics. For example, the displacement of disease-carrying animals to new geographic areas can introduce pathogens into susceptible human populations, leading to outbreaks and epidemics [30].

Addressing climate and ecological change is crucial as part of the global strategy to prevent and control emerging infectious diseases. Implementation of climate change adaptation and mitigation measures can reduce the spread of vector-borne and waterborne diseases. Furthermore, the protection of ecosystems and conservation of biodiversity are essential to avoid disruption of natural disease cycles. Rising temperatures, changes in precipitation patterns, and biodiversity loss can alter natural habitats, favoring the proliferation and spread of disease-causing microorganism. Furthermore, deforestation, urbanization, and ecosystem destruction can lead to closer contact between humans and animals, thus increasing the risk of emerging infectious disease transmission.

Climate and ecological change are closely linked to the emergence and spread of emerging infectious diseases. Changes in climate patterns, environmental degradation, and ecosystem disruption promote disease transmission and increase the risk of outbreaks. Addressing these challenges requires climate change adaptation and mitigation measures, as well as biodiversity conservation and public awareness. Interdisciplinary research and collaboration are key to understanding and tackling this complex issue.

2.4 Antimicrobial resistance

The ability of disease-causing microorganisms to develop resistance to existing antimicrobial drugs represents a serious challenge to public health. Drug resistance is a natural phenomenon that occurs when microorganisms such as bacteria, viruses, fungi, and parasites develop the ability to survive and multiply in the presence of antimicrobial agents that were originally effective in combating them. Continuous exposure and inappropriate use of antimicrobial drugs in humans, animals, and agriculture have accelerated the emergence and spread of drug resistance [31]. This resistance compromises the effectiveness of treatments and increases the burden of infectious diseases worldwide.

The relationship between drug resistance and emerging infectious diseases is complex and multifaceted. Antimicrobial resistance can increase the emergence of new diseases or the re-emergence of previously controlled diseases. When antimicrobial drugs are ineffective in treating an infection, the microorganism can spread and cause severe and difficult-to-treat illnesses. Additionally, resistance can lead to an increase in the duration of the disease, the burden of morbidity, and mortality [32]. A prominent example is antibiotic resistance, one of the most pressing issues in public health. Bacteria resistant to multiple antibiotics, known as multidrug-resistant bacteria or superbugs, have emerged and spread worldwide.

Antibiotic resistance has become a threat to global public health. It is estimated that each year, 2.8 million deaths occur due to infections resistant to antibiotics, and this figure is expected to rise to 10 million deaths by 2050. The countries reporting the highest number of cases of antibiotic-resistant bacteria are India, China, Brazil, Russia, and the United States of America. It is estimated that approximately 70% of Gram-negative bacteria and 30% of Gram-positive bacteria have developed some degree of antibiotic resistance. On the other hand, drugs with the highest antibiotic resistance are carbapenems, macrolides, and fluoroquinolones, which are used in the treatment of severe infections as meningitis, pneumonia, or tuberculosis.

Pathogens, such as methicillin-resistant Staphylococcus aureus (MRSA) and carbapenem-resistant Enterobacteriaceae (CRE), pose a significant threat to human health due to the limited availability of effective treatment options [33]. The enzyme NDM-1 (New Delhi metallo-beta-lactamase-1) causes resistance to nearly all antibiotics and initially emerged in India, from where it has spread to more than 120 countries. It took its name from the city (New Delhi) where the first infected patient is believed to have been, and from where it rapidly disseminated. It was found in drinking water in Delhi and in the upper reaches of the Ganges River. The resistance of Klebsiella pneumoniae to carbapenems increased from 2–52% in just 5 years. From 2008 to 2013, Escherichia coli became resistant in 83% to cephalosporins, broad-spectrum antibiotics, and in 85% to fluoroquinolones, a synthetic drug. For example, these bacteria, E. coli, present a 92% resistance to aminopenicillins (a group to which amoxicillin belongs) in India.

Natural selection plays a significant role in resistance development, as microorganisms with genetic traits that confer resistance have a survival advantage in an environment exposed to antimicrobial drugs [34]. The transmission of resistance genes can also occur between different species of microorganisms, further amplifying resistance spread.

Moreover, inappropriate use of antimicrobial drugs is a key factor in resistance development. Self-medication, unnecessary prescription, incorrect dosing, and premature treatment discontinuation can promote drug resistance. Additionally, the use of antimicrobials in agriculture, such as the use of antibiotics in intensive animal farming, also contributes to the emergence and spread of resistance [35, 36].

The issue of drug resistance and antimicrobial therapy has implications not only for human health but also for animal health and food safety. Drug resistance can be transmitted between humans and animals, leading to an increase in zoonotic diseases and greater difficulty in treating infections in both humans and animals. Furthermore, the presence of resistant microorganisms in food can pose a risk to public health if adequate control and prevention measures are not taken.

The emergence of emerging infectious diseases is also related to climate change and environmental degradation. Rising temperatures, changes in precipitation patterns, and biodiversity loss can alter natural habitats, favoring the proliferation and spread of disease-causing microorganisms. Additionally, deforestation, urbanization, and ecosystem destruction can lead to closer contact between humans and animals, thereby increasing the risk of emerging infectious disease transmission [13, 37].

Addressing the challenge of drug resistance and emerging infectious diseases requires concerted global action. It is necessary to strengthen epidemiological surveillance to monitor drug resistance and detect outbreaks of emerging diseases. Additionally, measures should be implemented to promote responsible use of antimicrobial drugs, including education and awareness regarding their proper use in both clinical and agricultural settings.

Research and development of new antimicrobial drugs are also crucial to tackle resistance. Investments in research and development are needed to discover new molecules and therapeutic approaches, as well as to foster innovation in the field of rapid and accurate diagnostics [38]. Furthermore, promoting international collaboration and cooperation among the human, animal, and environmental health sectors is necessary to comprehensively address the issue of drug resistance.

Drug resistance and antimicrobial therapy pose a significant threat to public health and are closely related to emerging infectious diseases. The emergence and spread of resistance compromise the effectiveness of treatments and increase the burden of infectious diseases worldwide. It is necessary to adopt a comprehensive approach that addresses drug resistance, including measures to promote responsible use of antimicrobial drugs, strengthen epidemiological surveillance, invest in research and development of new drugs, and foster international collaboration. Only through a multidisciplinary approach and coordinated action can we tackle this challenge and ensure the effectiveness of antimicrobial treatments in the future.

3. Characteristics of microorganisms causing emerging infectious diseases

Recent research highlights the importance of understanding the characteristics of microorganisms associated with emerging infectious diseases. Pathogenic microorganisms, such as bacteria, viruses, parasites, and fungi, can cause emerging infectious diseases with a significant impact on global public health and socioeconomic stability.

The microorganisms that cause emerging infectious diseases have several characteristics in common. Some of the characteristics of microorganisms include their ability to mutate and adapt, their capacity for quick replication, and their ability to overcome host defenses. Another important characteristic of microorganisms associated with emerging infectious diseases is their ability to efficiently transmit from one host to another.

Many of these microorganisms have a high capacity for propagation between human and animal populations. This means that they are capable of being transmitted from animals to humans, which we know as zoonotic diseases. Factors that favor this transmission would include environmental factors—degradation of the environment—close contact between humans and animals, the consumption of wild animals, and the expansion of intensive agriculture [39]. Recent decades have seen repeated pathogen emergence from wild or domestic animal reservoirs into human populations, from HIV-1 and HIV-2, to the 1918 influenza virus, to Middle East Respiratory Syndrome Coronavirus, to SARS-CoV-2 (Refs. [2, 3, 4]). For a novel pathogen to become a threat to human populations, first, contact between humans and the animal reservoir must occur; the pathogen must either have or evolve the capacity for human-to-human transmission and finally, this human-to-human transmission must enable expansion of the pathogen’s geographical range beyond the zone of spillover [40].

Unlike diseases such as HIV or highly transmissible coronaviruses, microorganisms that jump from animals to humans often have limited or no ability to transmit from person to person. In these cases, transmission occurs when humans come into direct contact with infected animals or their products, such as meat, milk, or excrement. This form of transmission is considered a “dead-end transmission” because there is no sustained spread of the microorganism within the human population. Importantly, although “dead-end” transmission of these microorganisms does not usually lead to epidemic outbreaks or pandemics, their individual and local consequences should not be underestimated. These zoonotic transmission events can cause severe and even lethal disease in cases where infection occurs in humans.

Throughout history, we have witnessed the emergence of various microorganisms that have posed a significant threat to global public health. These emerging microorganisms, including viruses, bacteria, fungi, and other pathogens, have emerged from different sources and have demonstrated their ability to cause severe diseases and spread rapidly. As we move further into the twenty-first century, it is crucial to be prepared to identify and respond to potential emerging microorganisms that may arise in the coming years.

One of the microorganisms that has generated significant concern in recent years is the Rift Valley Fever Virus (RVFV). This mosquito-borne virus primarily affects animals such as livestock and sheep but has shown the ability to infect humans and cause severe diseases, including fever, hemorrhage, and neurological problems. Although person-to-person transmission is still limited, RVFV outbreaks have caused high mortality and morbidity in regions of Africa and are considered a potential threat to global public health [41]. Another microorganism deserving attention is the avian influenza virus H₇N₉. This virus, primarily transmitted from birds to humans, has caused sporadic outbreaks in China since 2013. While sustained human-to-human transmission has not been observed thus far, human cases of H₇N₉ infection have been severe with a high mortality rate. We must remain vigilant about the possibility of this virus acquiring mutations that enable more efficient spread among humans, which would pose a significant threat to global public health. Additionally, we should also be attentive to Nipah viruses, such as the Nipah virus (NiV) and the Hendra virus (HeV). These zoonotic viruses have caused outbreaks in Southeast Asia and Australia, respectively. NiV has demonstrated the ability to spread from person to person in certain outbreaks, increasing concerns about its potential to trigger a large-scale epidemic. Nipah virus infections have been associated with a high mortality rate and severe symptoms such as encephalitis and pneumonia [42].

Furthermore, some bacteria also pose an emerging threat to public health. Carbapenem-resistant Klebsiella pneumoniae (KPC) and methicillin-resistant Staphylococcus aureus (MRSA) are prominent examples of multidrug-resistant bacteria. The resistance to antibiotics limits treatment options and presents a significant challenge for controlling infections caused by these bacteria [43]. When considering possible emerging microorganisms that could pose a threat to global public health in the coming years, it is essential to mention the Marburg virus. This virus belongs to the same family as the Ebola virus and has caused sporadic outbreaks in Central Africa since its discovery in 1967. Although Marburg fever outbreaks have been less frequent than Ebola, the high mortality rate associated with this disease (up to 90%) and the lack of specific treatments make it a significant global threat [44]. Another microorganism that has garnered attention is the Middle East Respiratory Syndrome Coronavirus (MERS-CoV) and various variants of coronaviruses. MERS-CoV was first identified in Saudi Arabia in 2012 and has caused sporadic outbreaks since then. Although person-to-person transmission is limited, cases of MERS-CoV infection have been severe, with high mortality rates. The concern lies in the possibility that this virus may undergo mutations that allow more efficient transmission among humans, potentially leading to a large-scale epidemic [45].

4. Impact of emerging infectious diseases on populations, public health, and the global economy

Emerging infectious diseases represent a constant challenge and danger to public health and the global economy. These diseases, caused by new pathogens or the emergence of resistant strains, have a high potential for transmission with significant impacts on populations and healthcare systems. The recent outbreak of SARS-CoV-2 has highlighted the threat posed by these emerging diseases to public health and the global economy. We must assess what the emergence of these new germs and diseases means for populations, global public health, and the economy at both regional and global levels. Table 2 shows the human and economic impact of major emerging diseases.

Year	Event	Estimated mortality and morbidity	Economic cost
1981	AIDS/HIV	70 million infections.; 37 million deaths [46]	3–4% of GDP in Africa [47]
2003	SARS	8100 cases and 744 deaths [48]	US $3 billions in China; US $6 billions in Canada; US $5 billions in Singapore [49]
2013	Ebola	20,646 cases; 11,323 deaths [50]	US$2 billion loss in Guinea, Liberia, and Sierra Leone [51]
2020	COVID	767.984.989 cases*; 6.943.390 deaths*; [52]	4,3% del GPD worldwide [53]

Table 2.

Impact of various emerging diseases on morbidity and mortality and their economic cost.

^*

June 2023.

The rapid spread of these diseases can result in epidemics or pandemics that affect entire communities. In addition to direct deaths caused by these diseases, there are serious consequences in terms of long-term morbidity and disability. Furthermore, these diseases can disproportionately impact vulnerable populations, such as children, the elderly, and people with chronic illnesses. According to a study conducted by Johnson et al. [54], it was found that the impact of emerging infectious diseases on the population can be particularly severe in areas with fragile healthcare systems and limited resources. Additionally, infectious diseases can negatively affect people’s quality of life, causing stress, anxiety, and an increase in disease burden.

4.1 Examples of the impact produced by EIDs

At the end of the late 1970s of the last century, the general perception was that the era of major epidemics had come to an end; we had made progress in treating bacterial diseases with the advent of antibiotics and vaccines, which offered broad protection against multiple viral diseases. However, the Human Immunodeficiency Virus (HIV) emerged and changed everything, along with the Acquired Immunodeficiency Syndrome (AIDS) associated with its infection. A zoonosis that encountered the perfect environment: mutations that made its transmission among humans more efficient, social changes associated with new sexual behaviors, and the emergence of parenteral heroin use. These factors amplified and facilitated its expansion.

Human Immunodeficiency Virus is a spherical retrovirus, equipped with an envelope and a protein capsid. Its genome is a single-stranded RNA chain that is temporarily copied to DNA in order to replicate and integrate into the genome of the infected cell. The protein antigens on the outer envelope specifically assemble with membrane proteins of susceptible cells, especially cluster of differentiation 4 (CD4) T cells. The process of converting RNA to DNA is a key feature of retroviruses and is carried out through enzymatic actions of reverse transcriptase. The Human Immunodeficiency Virus Type 1 (HIV-1), responsible for the current pandemic, has been closely related to Simian Immunodeficiency Virus (SIVcpz), which infects populations of the Central African subspecies of the common chimpanzee, while Human Immunodeficiency Virus Type 2 (HIV-2), on the other hand, originates from Simian Immunodeficiency Virus (SIVsm), which is found in the mangabey monkeys inhabiting the coastal forests from Senegal to Ivory Coast.

As a result, the emergence of HIV and its associated AIDS pandemic demonstrates how an EID can significantly impact global health. It highlights the importance of understanding the factors that contribute to the spread and adaptation of these pathogens to human populations. Furthermore, it serves as a reminder of the ongoing need for research, surveillance, and preparedness to effectively respond to the ever-changing landscape of infectious diseases.

It is estimated that 38.4 million people are living with HIV worldwide, and the annual number of deaths from HIV-related illnesses, which has decreased to below one million since 2017, still stands at around 700,000 deaths as of 2021. The accumulated incidence since the beginning of the pandemic is 85.6 million cases, with nearly 41 million infected individuals having died worldwide. The cost of HIV, both in terms of population and economics, has been enormous, particularly in African countries.

Until 2022, AIDS (Acquired Immunodeficiency Syndrome) has had a profound macroeconomic impact worldwide. This impact has been especially notable in regions with high rates of HIV infection, such as some parts of sub-Saharan Africa, but it has also affected both developed and developing economies. One of the primary aspects, though not the only one, of the economic cost of the infection’s impact has been the significant increase in healthcare expenses in affected countries. The costs associated with prevention, diagnosis, and treatment, including antiretroviral drugs and other medications, have been enormous. Additionally, treating HIV/AIDS-related complications has required considerable resources in terms of medical personnel, hospitals, and medical equipment. Moreover, the loss of human capital due to the death of productive-age individuals, particularly in sub-Saharan countries, has been devastating. AIDS has disproportionately affected young and working-age people, leading to imbalances in the population and depriving economies of valuable human resources for socioeconomic development.

On the other hand, AIDS has placed a significant burden on affected families and communities. The disease has led to the orphanhood of children, creating a need for more resources for the care and education of these orphans. It has also increased the caregiving burden for family members who have to attend to people with HIV/AIDS. The socioeconomic consequences of HIV/AIDS have been far-reaching, affecting not only individuals but also societies and economies as a whole.

Addressing the economic impact of HIV/AIDS requires a comprehensive approach, including continued investment in prevention, treatment, and support services. Ensuring access to antiretroviral therapy and other essential healthcare services for all those living with HIV is crucial to reducing the economic burden of the disease. Additionally, efforts to promote education and awareness about HIV transmission and prevention can help to reduce new infections and the associated economic costs. Supporting affected families and communities through social programs and initiatives is also vital to alleviate the broader socioeconomic consequences of HIV/AIDS. Only through concerted and sustained efforts can we effectively address the economic challenges posed by HIV/AIDS and work toward a future free from this devastating disease. It is estimated that by 2025, 29 billion US dollars (in US dollars of 2019) will be needed for the HIV response in low- and middle-income countries, including countries that were previously considered high-income countries, in order to end AIDS as a threat to global public health.

Coronaviruses are an extensive group of RNA viruses that can cause various conditions, ranging from the common cold to more severe diseases such as bronchitis, bronchiolitis, pneumonia, Middle East Respiratory Syndrome Coronavirus (MERS-CoV), or Severe Acute Respiratory Syndrome (SARS-CoV), among others. Toward the end of 2019, Chinese authorities reported an outbreak of a new coronavirus (SARS-CoV-2) in the city of Wuhan, originating from a zoonosis. To contain the SARS-CoV-2 outbreak, China imposed lockdowns and restricted population movements in several cities to prevent the virus’s spread, as the Chinese New Year triggers the world’s largest annual migration. By January 24, 2020, some sources indicated over 1300 identified cases, primarily in China, but also in the United States of America, France, Thailand, Japan, Nepal, and South Korea. On March 11, 2020, the World Health Organization (WHO) declared the SARS-CoV-2 outbreak a pandemic.

The SARS (Severe Acute Respiratory Syndrome) outbreak in 2002 and 2003 had a significant impact on the global economy. While the spread of SARS was quickly controlled and did not escalate into a large-scale pandemic like the current one, the macroeconomic cost of this outbreak was considerable (Table 2). During the SARS outbreak, there was a significant decrease in tourism, hospitality, and international travel in the affected regions. Countries and cities with SARS cases experienced a drastic reduction in foreign visitors and an increase in travel cancelations, resulting in revenue loss for the tourism industry and related services. The SARS outbreak created uncertainty among consumers and investors. Consumer confidence declined due to fear of the disease, leading to reduced spending and commercial activities. Investors also became cautious and hesitant to invest in the regions affected by the outbreak. Despite the relatively quick containment of the SARS outbreak compared to other emerging viruses, its macroeconomic consequences were notable. Global estimates of direct and indirect economic losses due to SARS vary in different reports, but the total cost is estimated to have ranged from several billions to tens of billions of dollars.

The MERS coronavirus, previously known as “novel coronaviru” “(nCoV), is a single-stranded RNA virus. It causes Middle East Respiratory Syndrome Coronavirus (MERS-CoV), first detected in Saudi Arabia in 2012. The origin of this virus is currently unknown but is believed to have likely originated from an animal source. The virus is now spreading to South Korea, raising concerns as coronaviruses can often mutate, potentially leading to a pandemic. All known cases have been linked to the Arabian Peninsula, either occurring there or in nearby countries. There are many uncertainties regarding the origin and transmission of MERS-CoV, which is typical of an emerging disease. Various possibilities, including environmental factors, animal sources, and human exposures, are being considered. As a result, the precise modes of transmission of the MERS coronavirus remain to be well defined.

Middle East Respiratory Syndrome outbreaks have resulted in temporary closures of medical facilities, hotels, and other businesses, leading to income loss in these sectors. Additionally, the implementation of control and prevention measures has resulted in restrictions on commercial activities, negatively affecting the local economy and companies operating in those regions.

The emergence of HIV in the last century, the Ebola virus disease epidemic in West Africa, the rise of antimicrobial-resistant pathogens, and the COVID-19 pandemic have reinforced the concept of Global Health. In addition to the impact these diseases have on population health, we must consider their far-reaching economic implications. Broader economic evaluations can provide a multisectoral translational perspective on the costs of the disease beyond approaches focused solely on human health, which consider disease cases, direct medical expenses, and public health functions and interventions [55]. In a study published by Fan et al. [56], it was estimated that the COVID-19 outbreak could have a significant impact on the global economy, with estimated economic losses in trillions of dollars. Additionally, the most affected countries may experience a decline in their gross domestic product (GDP) and an increase in public debt burden. These economic impacts can have long-lasting effects on communities and hinder long-term recovery.

The traditional approach to assessing the economic impact on health focuses on evaluating the direct costs of the disease burden (care, diagnosis, and treatment of the disease) and indirect costs (wages, productivity losses, etc.). However, this approach does not provide a comprehensive view of the economic impact of these diseases, such as the implementation of contagion control measures, lockdowns, trade closures, travel restrictions, etc., as we have seen during the early months of the SARS-CoV-2 pandemic, along with the collapse of healthcare systems leading to diagnostic delays and premature deaths associated with this fact [55]. The World Health Organization (WHO) has proposed a framework to calculate a broader economic impact of diseases [57] that allows for analyzing the economic consequences of biological threats by examining wider impacts, including human behavior, resilience speed, and the “fear factor.”

Let us take a look at some examples of the multisectoral economic impact of emerging infectious diseases (EIDs). As we mentioned earlier in this chapter, over 60% of EIDs have a zoonotic origin, which can result in significant economic impacts on the agricultural and livestock sectors. According to data provided by the World Organization for Animal Health [58], zoonotic diseases lead to a significant percentage of livestock culling as part of control strategies, along with a decline in livestock exports.

If we analyze the impact of COVID-19 on global agriculture, it has been significant and has affected various aspects of the food chain and agricultural production. First, there has been a major disruption in the food supply chain due to the pandemic, resulting in varying degrees of food supply disruptions depending on the country or region. This includes movement restrictions, logistical disruptions in distribution, and border closures. These factors have hindered the timely delivery of seeds, fertilizers, and pesticides, as well as the harvesting and distribution of agricultural products [59].

Second, movement restrictions and social distancing measures have limited the availability of agricultural labor, especially in countries that rely on migrant agricultural workers. This has affected the capacity for planting, harvesting, and crop maintenance, resulting in production losses and increased labor costs. Furthermore, the economic crisis associated with the pandemic, coupled with rising prices, has led to a decrease in demand and changes in consumption patterns, particularly in sectors such as horticulture and perishable products. Changes in consumption patterns, such as the closure of restaurants and a preference for nonperishable products, have also affected the marketing and prices of certain agricultural products, leading to a decline in demand.

During the SARS outbreak in 2003, tourism in Hong Kong dropped by 60%, and in Singapore by over 70%, with similar impacts observed in Mexico during the swine flu outbreak. However, possibly the greatest impact has been witnessed during the recent COVID-19 pandemic, which has had a significant impact on the tourism and airline industry. The measures implemented to contain the spread of the virus led to a massive decrease in travel and tourism demand. Airlines, hotels, tour operators, and other industry stakeholders experienced a drastic reduction in occupancy and revenue. The closure of tourism-related businesses and the decrease in travel demand have resulted in a substantial loss of jobs in the sector. Additionally, we should consider the significant decline in tourism revenues, which have had a significant impact on local and national economies, particularly in destinations heavily reliant on tourism [60, 61].

Furthermore, we should not underestimate the environmental impact that follows the emergence of EIDs. The damage to natural resources, loss of wildlife populations, and environmental pollution are often overlooked in economic assessments related to these diseases [55]. These pathogens, including viruses, bacteria, and other microorganisms, have the potential to cause outbreaks and pandemics resulting in considerable morbidity, mortality, and negative economic impact, with a potentially devastating effect on population health. Emerging infectious diseases pose a constant threat to healthcare systems worldwide. These pathogens can cause outbreaks and pandemics that test the responsiveness and resources of healthcare systems. One of the most feared consequences is the increased healthcare burden due to these diseases, which generate a rapid surge in demand for care and services, often within a short period of time. This will require a quick and effective response from healthcare systems, involving an increase in the number of consultations, hospitalizations, and intensive care. This additional burden can put pressure on available resources, resulting in a shortage of beds, medical personnel, and necessary supplies, to provide quality care. Moreover, control measures will be necessary to minimize the impact of these diseases, including contact tracing, case isolation, and other measures, which will require a rapid and effective mobilization of professional and material resources. All of this should be considered alongside the stress, fatigue from increased healthcare workload, and fear of contagion among healthcare personnel.

5. Strategies to address the impact of emerging infectious diseases

Emerging infectious diseases pose a significant threat to human, animal, and environmental health globally. Outbreaks of diseases, such as SARS-CoV-2, Ebola virus, avian flu, and Zika virus, among many others, have highlighted the need for a comprehensive and multidisciplinary approach to address these public health crises. Prevention and control of these diseases are crucial to reducing their impact on populations, public health, and the economy. Some key strategies include:

• Strengthenicnd resilient healthcare systems is essential, with the capacity to rapidly detect and respond to outbreaks of emerging infectious diseases. This involves improving healthcare infrastructure, training healthcare personnel, and strengthening epidemiological surveillance.

• The fight against antibiotic resistance must incorporate some important measures. It is essential to educate healthcare professionals, patients, and the general public about the proper use of antibiotics. Clear guidelines should be provided on when to prescribe them and when not to, as well as the importance of completing the treatment cycle. Prudent use of antibiotics is crucial. Antibiotics should be used only when necessary and for the treatment of bacterial infections. Additionally, it is crucial to establish surveillance systems to monitor antibiotic resistance and track usage patterns in different regions and populations. This will help identify emerging issues and take preventive measures (Centers for Disease Control and Prevention (CDC) and European Centre for Disease Prevention and Control (ECDC) programs) along with data exchange between countries to track drug resistance patterns. A prominent example of this is the Global Antimicrobial Resistance Surveillance System (GLASS). GLASS is an initiative led by the World Health Organization (WHO) that aims to collect, analyze, and share data on antimicrobial resistance worldwide. Through GLASS, countries can provide data on antibiotic resistance and antimicrobial use in humans and animals. These data are collected and analyzed globally to identify emerging resistance patterns, areas of concern, and trends over time.

• Research and development of vaccines and treatments: Investments in research and development are needed to accelerate the creation of effective vaccines and antiviral or antibiotic treatments. This includes supporting international collaboration and the exchange of data and samples.

• Enhancing surveillance and early detection: Strengthening epidemiological surveillance systems is necessary to quickly identify outbreaks of emerging infectious diseases and monitor their evolution. This involves implementing technology-based surveillance systems, improving healthcare staff training, and promoting collaboration between countries.

• Education and public communication: Promoting education and public awareness about emerging infectious diseases is essential. This includes providing accurate and timely information about these diseases, their modes of transmission, and preventive measures. Additionally, clear and transparent communication by health authorities is needed to build trust in prevention and control strategies.

• International cooperation: Cooperation and collaboration between countries are critical to addressing emerging infectious diseases globally. This includes sharing information, resources, and best practices, as well as coordinating efforts to prevent the spread of cross-border diseases. This concept is known as Global Health.

• Addressing associated factors: Effectively preventing emerging infectious diseases requires addressing underlying factors contributing to their emergence and spread. These factors include deforestation, environmental degradation, climate change, rapid urbanization, and antimicrobial resistance. Policies and measures must be implemented to tackle these challenges, including promoting sustainable agricultural practices, conserving natural ecosystems, and properly regulating the use of antimicrobials.

An outstanding example of the effective adaptation of specific policies in a strategy to minimize the impact of an emerging infection is the global response to the HIV/AIDS pandemic. Through collaboration among governments, international organizations, and civil society, prevention and treatment programs, access to antiretroviral drugs, and educational campaigns have been implemented, significantly reducing the impact of HIV/AIDS in many parts of the world. The fight against this disease has demonstrated that a coordinated and evidence-based response can make a difference in mitigating the impact of emerging infections.

Several countries and communities have demonstrated resilience and effective adaptation strategies against emerging infections. Let us look at some prominent examples.

During the COVID-19 pandemic, Taiwan implemented a rapid and efficient response to contain the virus spread. Their approach focused on early detection, contact tracing, isolating positive cases, and extensive use of information and communication technologies for monitoring and communication. This response has been widely praised for its effectiveness in containing the virus and minimizing its impact on the community. Singapore was another example of a country that has shown a quick and effective response to emerging infections. During the SARS pandemic in 2003 and the COVID-19 pandemic, Singapore implemented strict control measures, including contact tracing, quarantine, and social distancing. Clear and transparent communication with the public has also been a key component of their strategy. Uganda, on the other hand, has developed a comprehensive approach to address HIV/AIDS, which includes prevention, treatment, and healthcare programs for affected individuals. Additionally, they have worked to reduce the stigma and discrimination associated with the disease, which has contributed to improved access to care and support services. Finally, Brazil demonstrated an effective response to outbreaks of yellow fever and other infectious diseases. The country has carried out massive vaccination campaigns, improved epidemiological surveillance, and strengthened its healthcare system to address outbreaks promptly and in a coordinated manner.

These examples highlight the importance of a timely, coordinated, and evidence-based response to address emerging infections. Resilience and effective adaptation in these communities and countries have been crucial in minimizing the impact of infectious diseases and protecting public health.

5.1 The “One Health” approach and emerging infectious diseases

As mentioned earlier, emerging infectious diseases (EIDs) constitute a significant threat to global human health. The “One Health” approach is based on the recognition that human, animal, and environmental health are intrinsically interconnected. It proposes a holistic perspective that encompasses the interaction between humans, animals, plants, and the environment, acknowledging that public health challenges cannot be effectively addressed without close collaboration and integration of knowledge and efforts across disciplines.

The concept of “One Health” is not new and dates back at least 200 years [62]. The most commonly used definition shared by the Centers for Disease Control and Prevention (CDC) in the United States of America and the One Health Commission is: “One Health is defined as a collaborative, multisectoral, and transdisciplinary approach that works at the local, regional, national, and global levels to achieve optimal health outcomes by recognizing the interconnection between people, animals, plants, and their shared environment.” A simpler version of this definition is provided by the One Health Institute at the University of California, Davis: “One Health is an approach to ensure the well-being of people, animals, and the environment through collaborative problem-solving at the local, national, and global levels” [63].

In the context of EIDs, the “One Health” approach focuses on understanding the factors contributing to the emergence and spread of these diseases. This involves evaluating the links between human and animal health, monitoring diseases in animals and their potential transmission to humans, as well as assessing environmental factors that may influence disease spread. The “One Health” approach is crucial in understanding emerging infectious diseases due to several key factors.

Many of these diseases originate in animals and are transmitted to humans. Disease surveillance in animals, such as respiratory viruses in birds or rodent-borne viruses, is essential to identify potential transmission to humans. The “One Health” approach promotes collaboration between human and animal health professionals to monitor and prevent the spread of these diseases [64]. Furthermore, climate change and environmental degradation can influence the spread of emerging infectious diseases. Rising temperatures, deforestation, and urbanization can disrupt ecosystems and lead to increased contact between humans, animals, and pathogens. The “One Health” approach emphasizes the importance of assessing environmental impacts and climate change in the emergence and spread of emerging infectious diseases. It recognizes the need to understand the effects of climate change and environmental degradation on human and animal health and advocates for integrated management of these issues.

Antimicrobial resistance is becoming an increasing concern in the context of EIDs. Overuse and inappropriate use of antibiotics in humans and animals contribute to the development of resistance, making treatment more difficult. This approach promotes the implementation of policies and practices for responsible use of antimicrobials in both human and animal health. Lastly, early detection and rapid response are crucial in controlling EIDs. The “One Health” approach emphasizes the importance of establishing integrated surveillance systems that enable early detection of outbreaks in humans and animals, facilitating a rapid and effective response to contain the spread.

6. Conclusions

We can conclude by stating that emerging infectious diseases represent a significant threat to public health and global stability. As the world faces the emergence of new pathogens and unexpected outbreaks, it is crucial that we are prepared to respond quickly and effectively. These diseases have the potential to cause high morbidity and mortality, as well as trigger health and socioeconomic crises. We have witnessed how COVID-19 has affected millions of people worldwide, overwhelming healthcare systems, imposing mobility restrictions, and having a significant impact on the global economy.

Early detection, epidemiological surveillance, strengthening healthcare systems, and implementing preventive measures are key elements in responding to emerging infectious diseases. Scientific research, development of effective vaccines and treatments, as well as public education and awareness, also play a fundamental role in mitigating these outbreaks. Furthermore, it is essential to strengthen international cooperation and collaboration to jointly address these threats. Governments, international organizations, healthcare professionals, and society as a whole must come together to share information, resources, and best practices. Additionally, it is crucial that healthcare systems are prepared and flexible enough to meet the challenges posed by emerging infectious diseases. Addressing inequalities in access to healthcare and strengthening health systems in low-resource countries are also necessary, as emerging infectious diseases can have a particularly devastating impact in these regions. Learning from lessons of past outbreaks such as Ebola and SARS is crucial to improve our preparedness and response to future emerging infectious diseases. Cooperation between the public and private sectors, collaboration among countries, and global coordination are essential to effectively confront these challenges and protect the health of populations worldwide.

In summary, emerging infectious diseases constantly pose a threat to public health and global stability. Their impact on the population and the economy is significant, and a strong and coordinated global response is required. Investment in research, international cooperation, preparedness, and strengthening of healthcare systems, as well as public education and awareness, are key elements to address and mitigate the effects of these diseases. Only through a joint and collaborative approach can we be prepared to face the challenges presented by emerging infectious diseases. The “One Health” approach promotes multidisciplinary collaboration among professionals in human health, veterinary medicine, ecology, epidemiology, and other related disciplines. This collaboration allows for a better understanding of the factors contributing to the emergence and spread of emerging infectious diseases, as well as the development of more effective preventive and control strategies.

Conflict of interest

The author declares no conflict of interest.