What are the different types of pneumonia?

Types of Pneumonia

Pneumonia is classified into distinct types based primarily on the setting of acquisition (community vs. hospital), and secondarily by causative pathogen (bacterial, viral, fungal, or atypical organisms). 1

Classification by Setting of Acquisition

Community-Acquired Pneumonia (CAP)

• CAP is defined as pneumonia occurring in a previously healthy individual from an infection acquired outside the hospital setting. 2, 1
• This represents the most common form of pneumonia and accounts for up to 5.6 million cases annually in the United States, with up to 1.1 million requiring hospitalization. 1
• CAP is the sixth leading cause of death and the number one cause of death from infectious diseases in the United States. 1

Hospital-Acquired (Nosocomial) Pneumonia

• Hospital-acquired pneumonia develops after 48 hours of hospitalization and was not present at admission. 2, 1
• This is the second most common nosocomial infection after bloodstream infections, and the most common infection acquired in intensive care units. 2
• The pathogen spectrum differs significantly from CAP, with gram-negative bacilli (including Pseudomonas aeruginosa, Klebsiella pneumoniae, E. coli) predominating. 2, 3

Ventilator-Associated Pneumonia (VAP)

• VAP is a subset of hospital-acquired pneumonia occurring in mechanically ventilated patients, affecting up to 12% of ventilated children. 2, 1
• Pseudomonas aeruginosa is the most frequently isolated pathogen in VAP, especially with prolonged mechanical ventilation. 3
• Polymicrobial infections occur in 40% of ventilated patients, with particularly high frequency in those with acute respiratory distress syndrome. 3

Classification by Causative Pathogen

Bacterial Pneumonia

Typical Bacterial Pathogens

• Streptococcus pneumoniae remains the predominant cause of bacterial CAP, accounting for 50-90% of cases in adults. 2, 1
• Haemophilus influenzae is the second most common bacterial pathogen in CAP. 2
• Staphylococcus aureus causes severe pneumonia, particularly in ICU-admitted patients and those with hospital-acquired infection. 2
• Gram-negative enteric bacilli (Klebsiella pneumoniae, E. coli, Enterobacter species) are important causes, particularly in patients with specific risk factors including alcoholism, chronic lung disease, and recent antibiotic use. 2, 4, 3

Atypical Bacterial Pathogens

• Mycoplasma pneumoniae causes atypical pneumonia characterized by slow progression, malaise, and low-grade fever, accounting for 8-16% of hospitalizations in school-aged children and young adolescents. 2
• Chlamydia pneumoniae and Legionella species are recognized atypical pathogens, with Legionella being particularly important in severe CAP requiring ICU admission. 2
• These organisms cannot be reliably distinguished from typical bacterial pneumonia by chest radiography alone. 2

Anaerobic Pneumonia (Aspiration)

• Aspiration pneumonia involves normal oropharyngeal aerobic and anaerobic flora, particularly in patients with risk factors including alcoholism with loss of consciousness, poor dentition, and altered mental status. 2, 4
• Anaerobic coverage is specifically indicated in patients with classic aspiration pleuropulmonary syndrome. 4

Viral Pneumonia

• Viral pathogens, particularly respiratory syncytial virus, are the most common cause of pneumonia in infants, toddlers, and preschool-aged children. 2, 1
• Influenza virus is the most important cause of viral pneumonia in adults. 2
• Other important viral pathogens include adenovirus and, since 2019, SARS-CoV-2 (COVID-19), which has distinct features including olfactory/gustatory dysfunction, lymphopenia, and characteristic imaging appearance. 5

Fungal Pneumonia

• Endemic fungi (Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis) should be considered in patients with travel history to endemic areas. 6
• Opportunistic fungal pneumonias (including Candida, Aspergillus, Phycomycetes species) occur primarily in immunocompromised hosts. 6, 7
• Pneumocystis carinii (now Pneumocystis jirovecii) is an important consideration in HIV-infected patients and other immunosuppressed individuals. 2

Mycobacterial Pneumonia

• Mycobacterium tuberculosis must always be considered, particularly in patients with specific risk factors including alcoholism, immunosuppression, residence in endemic areas, or nursing home residence. 2, 4

Age-Specific Pathogen Patterns

Infants, Toddlers, and Preschool Children

• Viruses are the most common pathogens, with respiratory syncytial virus being the usual culprit. 2
• When bacterial pneumonia occurs, S. pneumoniae is most common, though prevalence is decreasing due to pneumococcal vaccination. 2

School-Aged Children and Young Adolescents

• Bacterial pneumonia becomes more common, with S. pneumoniae remaining the most frequent pathogen. 2
• M. pneumoniae accounts for 8-16% of hospitalizations in this age group. 2

Adults

• S. pneumoniae is the predominant pathogen across all adult age groups in CAP. 2, 1
• The pathogen spectrum expands in older adults with comorbidities to include gram-negative bacilli and mixed infections. 2

Severe Pneumonia Requiring ICU Admission

Patients admitted to the ICU with respiratory infection have severe pneumonia with mortality rates up to 50%, requiring immediate recognition and broader antimicrobial coverage. 2

Pathogens in Severe CAP (Without Pseudomonas Risk)

• S. pneumoniae (including drug-resistant strains), Legionella species, H. influenzae, enteric gram-negative bacilli, S. aureus, M. pneumoniae, respiratory viruses, C. pneumoniae, M. tuberculosis, and endemic fungi. 2

Severe CAP with Pseudomonas Risk Factors

• P. aeruginosa should be considered only when specific risk factors are present, including chronic or prolonged broad-spectrum antibiotic therapy (≥7 days within the past month), structural lung disease, or recent healthcare exposure. 2, 3

Common Pitfalls

• Do not rely on chest radiography alone to distinguish viral from bacterial pneumonia or to identify specific bacterial pathogens—clinical context and risk factors are essential. 2
• Avoid sequential courses of different antibacterial agents covering the same pathogen spectrum in non-responding pneumonia; instead, escalate diagnostic efforts with bronchoscopy or biopsy to identify fungal or unusual pathogens. 7
• Even with comprehensive diagnostic testing including invasive procedures, a specific etiologic agent is identified in only approximately 50% of pneumonia cases. 8
• Consider multidrug-resistant pathogens when patients have antimicrobial therapy in the preceding 90 days, hospitalization ≥5 days, recent hospitalization, nursing home residence, chronic dialysis, or immunosuppression. 3