Pneumonia - Community-Acquired

Definition

Community-acquired pneumonia (CAP) is an acute infection of the pulmonary parenchyma acquired outside of hospitals or long-term care facilities, associated with symptoms such as fever, cough, sputum production, dyspnea, and radiographic evidence of an infiltrate.

Epidemiology

• One of the leading causes of infectious mortality worldwide.
• More common in elderly patients, smokers, and those with chronic illnesses (COPD, diabetes, heart disease).
• Incidence: approximately 5–10 cases per 1,000 adults annually.
• Mortality higher in hospitalized or ICU patients.
• Peak incidence during winter months.

Etiology

• Bacterial pathogens: Streptococcus pneumoniae (most common), Haemophilus influenzae, Staphylococcus aureus, Moraxella catarrhalis.
• Atypical bacteria: Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella pneumophila.
• Viral causes: Influenza virus, RSV, SARS-CoV-2, parainfluenza virus, adenovirus.
• Risk factors: smoking, chronic pulmonary disease, alcoholism, immunosuppression, aspiration, poor oral hygiene.

Pathophysiology

• Inhalation or aspiration of pathogenic organisms into alveoli.
• Failure of host defense mechanisms allows microbial proliferation.
• Inflammatory response leads to alveolar filling with exudate, impairing gas exchange.
• Cytokine release causes fever, leukocytosis, and systemic symptoms.
• Severe inflammation may lead to sepsis, respiratory failure, or multi-organ dysfunction.