Definition of pneumonia
Pneumonia is an infection of the lung parenchyma (the alveoli and surrounding tissue) caused by bacteria, viruses, fungi, or other organisms. It leads to inflammation and consolidation of lung tissue, impairing gas exchange. Common symptoms include fever, cough with sputum, dyspnea, and pleuritic chest pain.
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Here is the definition of pneumonia drawn from authoritative medical textbooks:
Pneumonia - Definition
Pneumonia is an infection of the lungs that leads to consolidation of the usually air-filled alveoli. It can occur in all age groups and is caused by a wide range of organisms including viruses, bacteria, mycobacteria, mycoplasma, and fungi.
- Textbook of Family Medicine, 9e, p. 321
Key Points from the Definition
- The core pathological event is consolidation - the alveoli (normally air-filled spaces) become filled with inflammatory exudate, fluid, or debris, impairing gas exchange.
- The term "pneumonia" refers specifically to infection of the lung parenchyma, distinguishing it from bronchitis (airways only) or pleuritis (pleural lining).
- It is not a single disease but a syndrome produced by many different organisms.
Causative Agents
| Category | Examples |
|---|---|
| Bacteria | Streptococcus pneumoniae, Haemophilus influenzae, Pseudomonas, Klebsiella |
| Atypical bacteria | Mycoplasma, Chlamydia, Legionella spp. |
| Viruses | Influenza A/B, RSV, SARS-CoV-2 |
| Fungi | Pneumocystis jirovecii (PCP), Coccidioides, Cryptococcus |
| Mycobacteria | Mycobacterium tuberculosis, NTM |
Classification
Pneumonia is further classified by setting and host, because these determine likely pathogens and management:
- Community-Acquired Pneumonia (CAP) - acquired outside the hospital
- Hospital-Acquired (Nosocomial) Pneumonia (HAP) - onset ≥48 hours after hospital admission
- Health Care-Associated Pneumonia (HCAP) - in patients from nursing homes or with frequent healthcare exposure
- Ventilator-Associated Pneumonia (VAP) - occurring in mechanically ventilated patients
- Opportunistic Pneumonia - in immunocompromised hosts (HIV, transplant, chemotherapy)
Clinical Hallmarks
- Cough (productive or non-productive, possibly blood-tinged)
- Fever and chills
- Dyspnea (shortness of breath)
- Pleuritic chest pain
- On exam: decreased breath sounds, dullness to percussion, egophony over the affected area
- Elevated WBC (>15,000/mm³) with neutrophil predominance in bacterial cases
Systemic viral infections (e.g., influenza, measles, varicella) can also trigger secondary bacterial pneumonia, highlighting the overlap between viral and bacterial causes.
Source: Textbook of Family Medicine 9e, p. 321 - "Pneumonia is an infection of the lungs that leads to consolidation of the usually air-filled alveoli. It occurs in all age groups and can be caused by various agents, including viruses, bacteria, mycobacteria, mycoplasma, and fungi."
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