Pneumonia
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"community-acquired pneumonia"[MeSH Terms] AND "etiology"[MeSH Terms]
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I now have comprehensive content from Goldman-Cecil covering epidemiology, pathobiology, risk factors, and causative organisms. Let me compile the full overview.
Community-Acquired Pneumonia (CAP): Overview
Definition
Pneumonia is an acute infection of the lung parenchyma associated with at least one respiratory symptom (cough, dyspnea, or chest pain) and abnormal opacities on lung imaging, with or without fever, in the absence of an alternative diagnosis. Community-acquired pneumonia (CAP) refers to pneumonia acquired outside of a healthcare setting. - Goldman-Cecil Medicine, p. 992
Epidemiology
- One of the most serious health conditions and the most common infectious cause of hospitalization and death in the United States.
- Accounts for ~7 annual healthcare visits per 1,000 young adults, but 96 per 1,000 in adults aged 85+.
- Hospitalization rates rise exponentially with age: 1-2/1,000 in young adults vs. almost 40/1,000 in persons aged 85+.
- ~35% of hospitalizations occur in patients with recent healthcare exposure; ~25% in immunocompromised patients.
- Cases peak November through March, coinciding with influenza and respiratory virus season.
- SARS-CoV-2 became the leading infectious cause of death during the COVID-19 pandemic, largely through pneumonia and respiratory failure. - Goldman-Cecil Medicine, pp. 990-993
Risk factors include: male sex, smoking, poor dental hygiene, crowded living conditions, malnutrition, alcohol use disorder, chronic immunosuppression, chronic lung disease, neurologic disease (impaired gag reflex), and use of opioids, proton pump inhibitors, or corticosteroids.
Types / Classification
Pneumonia is broadly classified by setting of acquisition and causative organism:
| Type | Definition |
|---|---|
| Community-Acquired (CAP) | Acquired in the community, in a non-hospital setting |
| Hospital-Acquired (HAP) | Develops ≥48 hours after hospital admission |
| Ventilator-Associated (VAP) | Develops ≥48 hours after endotracheal intubation |
| Immunocompromised Pneumonia | Includes both typical and atypical/opportunistic pathogens |
By anatomical pattern (radiologic/pathologic):
- Lobar pneumonia - consolidation of an entire lobe (classic pneumococcal)
- Bronchopneumonia - patchy, multifocal infiltrates around airways
- Interstitial pneumonia - diffuse infiltrates along alveolar walls (typical of atypical pathogens, viruses)
Causative Pathogens
Typical bacteria cause about 30% of cases. Viruses historically cause 20-30% (higher during COVID-19). In over 50% of cases, no clear pathogen is confirmed despite testing. - Goldman-Cecil Medicine, p. 993
| Patient Category | Common Pathogens |
|---|---|
| Outpatient, no comorbidities | S. pneumoniae, M. pneumoniae, C. pneumoniae, H. influenzae, respiratory viruses |
| Outpatient, with cardiopulmonary disease | All above + drug-resistant S. pneumoniae (DRSP), enteric Gram-negatives, anaerobes (aspiration) |
| Inpatient, with comorbidities | S. pneumoniae (incl. DRSP), H. influenzae, M. pneumoniae, C. pneumoniae, enteric Gram-negatives (incl. P. aeruginosa), viruses, Legionella, anaerobes |
| Severe CAP, no P. aeruginosa risk | S. pneumoniae, Legionella, H. influenzae, enteric GNBs, S. aureus, M. pneumoniae, viruses |
| Severe CAP, with P. aeruginosa risk | All above + P. aeruginosa |
- Fishman's Pulmonary Diseases, Table 125-2, p. 2202
Notable atypical pathogens: Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella spp. account for <5% of confirmed cases individually but are clinically significant due to atypical presentation and antibiotic requirements.
CA-MRSA: Rare but causes a severe, necrotizing form of CAP, often following influenza.
Pathobiology
Even in healthy lungs, continuous microaspiration of oropharyngeal flora occurs. The healthy lung microbiome is dominated by Prevotella, Veillonella, and Streptococcus species. Pneumonia develops when the homeostatic balance is disrupted - allowing one or more pathogens to become dominant and trigger a host inflammatory response.
Three factors regulate microbial burden in the lung:
- Immigration - aspiration, inhalation
- Elimination - mucociliary clearance, cough, innate/adaptive immunity
- Relative reproduction rates - shaped by pH, oxygen tension, temperature, surfactant
Conditions that promote pneumonia:
- Increased oropharyngeal colonization (aspiration risk, supine position, GERD)
- Decreased elimination (impaired cough, ciliary dysfunction, airway obstruction)
- Altered microenvironment (immunosuppression, tissue destruction)
Aging is the strongest single risk factor - it impairs stem cell reservoirs, mitochondrial function, mucociliary clearance, and upregulates epithelial adhesion receptors that facilitate bacterial colonization. - Goldman-Cecil Medicine, p. 993
Key Clinical Summary
| Feature | Details |
|---|---|
| Mortality (hospitalized) | ~6% in-hospital; ~15% at 1 month |
| Most common complication | Empyema (~3-5% of cases) |
| Cardiac complications | AF, new/worsening heart failure, MI in ~20-25% of hospitalized patients |
| Prevention | Pneumococcal vaccine, influenza vaccine, COVID-19 vaccine |
A Note on Recent Evidence
A PubMed search for recent systematic reviews on CAP etiology returned no results for the past 2 years with those filters - the Goldman-Cecil and Fishman's content above (IDSA/ATS 2019 guidelines-concordant) represents the current standard. No contradicting landmark evidence was identified.
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