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Introductory Chapter: Pneumonia

Written By

Aysan Moeinafshar and Nima Rezaei

Published: 11 May 2022

DOI: 10.5772/intechopen.103675

From the Edited Volume


Edited by Nima Rezaei

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1. Introduction

Pneumonia is an umbrella term regarding a variety of syndromes with different etiologies and refers to infection of the lung parenchyma. Pneumonia can be caused by a variety of microorganisms including bacterial, viral, and fungal pathogens [1]. Bacterial pneumonia can be divided into typical and atypical infections. Typical pneumonia is caused by microorganisms with the possibility of culturing on standard media or observation using gram staining techniques, such as Streptococcus pneumonia, Staphylococcus aureus, Haemophylus influenza, Moraxella catarrhalis, Group A streptococci, and gram-negative bacteria (both anaerobic and aerobic species). Atypical pneumonia is caused by pathogens that do not fit the aforementioned criteria; such as Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella [2]. A variety of viruses, of both RNA and DNA virus families, can lead to pneumonia characteristics in patients. Some examples of these viruses include respiratory syncytial virus (RSV), rhinovirus, influenza viruses, parainfluenza viruses, adenovirus, varicella-zoster virus (VZV), cytomegalovirus (CMV), especially in HIV-infected patients, measles, and coronavirus family [3]. Fungal infections, though mostly overlooked, are important sources of pneumonia in immunocompromised patients. Some of the important organisms in this group include Histoplasma, Blastomyces, and Coccidioides [4].


2. Types of pneumonia

Pneumonia is classified into three groups based on etiology, disease characteristics, and clinical setting of the pathogen transmission. These subtypes include community-acquired pneumonia (CAP), hospital-acquired pneumonia (HAP), and ventilator-associated pneumonia (VAP) [1].

CAP is a type of pneumonia that is acquired in a community setting, caused by both atypical and typical bacterial organisms, viruses, and fungi [1, 5]. HAP is considered a type of pneumonia acquired 48 h after hospital admission, with no incubation at time of admission [6]. On the other hand, VAP is a pneumonia acquired in patients under endotracheal incubation 48 h after the procedure [7]. The underlying pathogens responsible for HAP and VAP include gram-negative bacilli (Escherichia coli, Pseudomonas Aeruginosa, Acinetobacter, Enterobacter, Klebsiella, etc.) and gram-positive cocci such as S. aureus [8]. Also, aspiration of both small (micro-aspiration) and large (macro-aspiration) amounts of oropharyngeal and upper gastrointestinal secretions is responsible for aspiration pneumonia, which accounts for approximately 5–15% of CAP cases [9].

Pneumonia’s pattern of pulmonary involvement in these infectious diseases varies widely and can be categorized into lobar pneumonia, lobular pneumonia, and focal/diffused interstitial pneumonia [10].

Risk factors predisposing patients to each of these pneumonia subtypes are summarized in Table 1.

Pneumonia subtypeCAPHAPAspiration pneumonia
Risk factors
  • Age < 5

  • Age > 65 + comorbidities

  • Male gender

  • Immunocompromised

  • Life style (smoking, etc.)

  • Prematurityp

  • Household air pollutionp

  • Ambient particulate matterp

  • Suboptimal breast feedingp

  • Pulmonary diseaseA

  • DMA

  • CVDA

  • Chronic liver diseaseA

  • Male gender

  • Burns, trauma, surgery

  • History of antibiotic therapy

  • Malnutrition

  • Disease severity

  • Virulent pathogens in oropharynx

  • Pulmonary aspiration (and predisposing conditions to pulmonary aspiration)

  • ARDS

  • Impaired swallowing

  • Decreased consciousness

  • Impaired cough reflex


Table 1.

Rrisk factors of pneumonia. (DM = diabetes mellitus, CVD = cardiovascular disease, ARDS = acute respiratory distress syndrome, p superscript: pediatric cases, A superscript: adult cases).


3. Disease burden

The results of the Etiology of Pneumonia in the Community (EPIC) study in the united states indicated the annual incidence of CAP to be 2.4 cases per 1000 adults, mostly in age groups of >65 years old [12]. Similar studies in Europe estimated the annual incidence to be 1.07–1.2 cases per 1000 people [13]. The annual incidence of HAP is about 5–10 patients per 1000 hospital admissions worldwide and VAP cases include 10–25% of patients under ventilation [14].

Mortality of CAP in outpatient care, hospital wards, and ICU is <1%, 4–18%, and up to 50% respectively [15, 16, 17]. HAP and VAP are the most common causes of death in hospital-acquired infections with global mortality rates of 20–10% and 20–50% respectively [18, 19, 20, 21, 22].


4. Pathophysiology

Inability of the immune system in clearance of pathogens from the lower respiratory system is the basis for the incidence of pneumonia [23]. In addition to the pathogens, both local and systemic immune responses lead to parenchymal damage, constitutional symptoms, fluid congestion, pus formation in lungs, and reduction in alveolar compliance [24].

Pathological findings throughout this process consist of four stages including congestion, due to intra-alveolar edema, red hepatization, gray hepatization, both with characteristics of increased firmness of the parenchyma, and resolution [25].


5. Diagnosis

Most important symptoms stated by patients include fever, chills, diaphoresis, fatigue, myalgia, malaise, productive or non-productive coughs, dyspnea, and pleuritic chest pain [23]. Clinical data used in the definition of pneumonia include symptoms of acute lower respiratory tract disease for periods of less than 7 days, minimum of one systemic symptom, and newly appeared signs in chest examination unexplained by other conditions. Along with clinical criteria, laboratory tests, such as complete blood count (CBC), serum electrolytes, procalcitonin levels, renal and liver function tests, and radiologic findings, such as consolidations, abnormal silhouettes, opacities, and infiltrates, are also of importance in defining pneumonia [26].


6. Management

Management of CAP is carried out after a risk stratification process mostly using CURB-65 criteria. This scale consists of five criteria; Confusion, Uremia (BUN>20 mg/dl), respiratory rate > 30/minute, blood pressure < 90/60 mmHg, and ages>65 years. Treatments of HAP and VAP take more time and are more complicated. First-line treatment of pneumonia includes empirical antibiotic therapy using broad-spectrum antibiotics [1].

Figure 1 summarizes the approaches in the management of pneumonia.

Figure 1.

Approach to pneumonia [1].


  1. 1. Jain V, Vashisht R, Yilmaz G, Bhardwaj A. Pneumonia Pathology. FL: Treasure Island; 2021
  2. 2. Sattar SBA, Sharma S. Bacterial Pneumonia. FL: Treasure Island; 2021
  3. 3. Freeman AM, Leigh Townes RJ. Viral Pneumonia. FL: Treasure Island; 2021
  4. 4. Hage CA, Knox KS, Wheat LJ. Endemic mycoses: Overlooked causes of community acquired pneumonia. Respiratory Medicine. 2012;106(6):769-776
  5. 5. Mandell LA, Wunderink RG, Anzueto A, Bartlett JG, Campbell GD, Dean NC, et al. Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults. Clinical Infectious Diseases: An Offical Publication Infectious Diseases Socical American. 2007;44(Suppl 2):S27-S72
  6. 6. Kalil AC, Metersky ML, Klompas M, Muscedere J, Sweeney DA, Palmer LB, et al. Executive summary: Management of adults with hospital-acquired and ventilator-associated pneumonia: 2016 clinical practice guidelines by the Infectious Diseases Society of America and the American Thoracic Society. Clinical Infectious Diseases: An Offical Publication Infectious Diseases Socical American. 2016;63(5):575-582
  7. 7. Canadian Critical Care Trials Group. A randomized trial of diagnostic techniques for ventilator-associated pneumonia. The New England Journal of Medicine. Dec 2006;355(25):2619-2630
  8. 8. Jones RN. Microbial etiologies of hospital-acquired bacterial pneumonia and ventilator-associated bacterial pneumonia. Clinical Infectious Diseases: An Offical Publication Infectious Diseases Socical American. 2010;51(Suppl 1):S81-S87
  9. 9. Mandell LA, Niederman MS. Aspiration pneumonia. The New England Journal of Medicine. 2019;380(7):651-663
  10. 10. Gharib AM, Stern EJ. Radiology of pneumonia. The Medical Clinics of North America. 2001;85(6):1461-1491
  11. 11. Torres A, Cilloniz C, Niederman MS, Menéndez R, Chalmers JD, Wunderink RG, et al. Pneumonia. Nature Reviews Disease Primers [Internet]. 2021;7(1):25. DOI: 10.1038/s41572-021-00259-0
  12. 12. Jain S, Self WH, Wunderink RG, Fakhran S, Balk R, Bramley AM, et al. Community-acquired pneumonia requiring hospitalization among US adults. The New England Journal of Medicine. 2015;373(5):415-427
  13. 13. Torres A, Peetermans WE, Viegi G, Blasi F. Risk factors for community-acquired pneumonia in adults in Europe: A literature review. Thorax. 2013;68(11):1057-1065
  14. 14. Torres A, Niederman MS, Chastre J, Ewig S, Fernandez-Vandellos P, Hanberger H, et al. International ERS/ESICM/ESCMID/ALAT guidelines for the management of hospital-acquired pneumonia and ventilator-associated pneumonia: Guidelines for the management of hospital-acquired pneumonia (HAP)/ventilator-associated pneumonia (VAP) of the European Respiratory Society (ERS), European Society of Intensive Care Medicine (ESICM), European Society of Clinical Microbiology and Infectious Diseases (ESCMID) and Asociación Latinoamericana del Tórax (ALAT). The European Respiratory Journal. 2017;50(3):1700582
  15. 15. Arnold FW, Wiemken TL, Peyrani P, Ramirez JA, Brock GN. Mortality differences among hospitalized patients with community-acquired pneumonia in three world regions: Results from the Community-Acquired Pneumonia Organization (CAPO) International Cohort Study. Respiratory Medicine. 2013;107(7):1101-1111
  16. 16. Heo JY, Song JY. Disease burden and etiologic distribution of community-acquired pneumonia in adults: Evolving epidemiology in the era of pneumococcal conjugate vaccines. Infection & Chemotherapy. 2018;50(4):287-300
  17. 17. Cillóniz C, Ewig S, Polverino E, Marcos MA, Prina E, Sellares J, et al. Community-acquired pneumonia in outpatients: Aetiology and outcomes. The European Respiratory Journal. 2012;40(4):931-938
  18. 18. Magill SS, Edwards JR, Bamberg W, Beldavs ZG, Dumyati G, Kainer MA, et al. Multistate point-prevalence survey of health care–associated infections. The New England Journal of Medicine. 2014;370(13):1198-1208
  19. 19. Micek ST, Chew B, Hampton N, Kollef MH. A case-control study assessing the impact of nonventilated hospital-acquired pneumonia on patient outcomes. Chest. 2016;150(5):1008-1014
  20. 20. Melsen WG, Rovers MM, Groenwold RHH, Bergmans DCJJ, Camus C, Bauer TT, et al. Attributable mortality of ventilator-associated pneumonia: A meta-analysis of individual patient data from randomised prevention studies. The Lancet Infectious Diseases. 2013;13(8):665-671
  21. 21. Barbier F, Andremont A, Wolff M, Bouadma L. Hospital-acquired pneumonia and ventilator-associated pneumonia: Recent advances in epidemiology and management. Current Opinion in Pulmonary Medicine. 2013;19(3):216-228
  22. 22. Bassetti M, Righi E, Vena A, Graziano E, Russo A, Peghin M. Risk stratification and treatment of ICU-acquired pneumonia caused by multidrug-resistant/extensively drug-resistant/pandrug-resistant bacteria. Current Opinion in Critical Care. 2018;24(5):385-393
  23. 23. Kaysin A, Viera AJ. Community-acquired pneumonia in adults: Diagnosis and management. American Family Physician. 2016;94(9):698-706
  24. 24. Cilloniz C, Martin-Loeches I, Garcia-Vidal C, San Jose A, Torres A. Microbial etiology of pneumonia: Epidemiology, diagnosis and resistance patterns. International Journal of Molecular Sciences. Dec 2016;17(12):2120
  25. 25. Kradin RL, Digumarthy S. The pathology of pulmonary bacterial infection. Seminars in Diagnostic Pathology. 2017;34(6):498-509
  26. 26. Ticona JH, Zaccone VM, McFarlane IM. Community-acquired pneumonia: A focused review. American Journal of Medical Case Reports. 2021;9(1):45-52. Available from: [Accessed: April 11, 2020]

Written By

Aysan Moeinafshar and Nima Rezaei

Published: 11 May 2022