Community-acquired pneumonia (CAP) results from pathogen entry into the lower respiratory tract overwhelming local immune defenses. S. pneumoniae remains the most common typical bacterial cause, producing lobar consolidation with inflammatory exudate filling alveoli. Atypical organisms (Mycoplasma, Chlamydophila, Legionella) cause interstitial inflammation with patchy infiltrates. Hospital-acquired pneumonia (HAP, >= 48 hours after admission) and ventilator-associated pneumonia (VAP, >= 48 hours after intubation) involve more resistant organisms: MRSA, Pseudomonas aeruginosa, Acinetobacter, extended-spectrum beta-lactamase (ESBL) producers. Risk factors for multidrug-resistant (MDR) pathogens include IV antibiotics within 90 days, > 5 days hospitalization, prior MDR colonization, structural lung disease, and immunosuppression. Severity assessment using CRB-65 (outpatient) or CURB-65 and PSI/PORT (inpatient) guides site-of-care and empiric antibiotic decisions.