Pneumonia is an acute infection of the pulmonary parenchyma — the alveoli and interstitium — that produces inflammation, alveolar exudate, and impaired gas exchange. It is a leading cause of infection-related illness and death in the United States, with the greatest risk in older adults, immunocompromised people, and those with underlying lung disease, heart failure, or diabetes. Because the lower airways are normally sterile, pneumonia develops when the infectious burden overwhelms host defenses such as mucociliary clearance, the cough reflex, and alveolar macrophages, or when those defenses are impaired. Pneumonia is classified by where and how it is acquired, which predicts the likely organisms and the antibiotic approach: - Community-acquired pneumonia (CAP): develops outside a healthcare facility. The most common pathogen is Streptococcus pneumoniae, followed by Haemophilus influenzae and atypical organisms such as Mycoplasma, Chlamydophila, and Legionella. - Hospital-acquired (nosocomial) pneumonia (HAP): develops at least 48 hours after admission, with a higher risk of drug-resistant organisms such as MRSA and Pseudomonas aeruginosa. - Ventilator-associated pneumonia (VAP): HAP that develops at least 48 hours after endotracheal intubation. - Aspiration pneumonia: caused...