Pneumonia is an infection of the lung parenchyma causing inflammation, alveolar exudate accumulation, and consolidation. The pathological pattern differs by type: (1) Lobar pneumonia (typically Streptococcus pneumoniae) involves consolidation of an entire lobe, progressing through four stages - congestion (vascular engorgement, serous exudate), red hepatization (RBCs and neutrophils fill alveoli, lung resembles liver), gray hepatization (fibrin and degrading neutrophils), and resolution (enzymatic digestion of exudate). (2) Bronchopneumonia (patchy, multifocal) involves inflammation centered around bronchioles spreading to adjacent alveoli, commonly caused by Staphylococcus aureus, Haemophilus influenzae, and gram-negative organisms. Classification by setting: Community-acquired pneumonia (CAP) develops outside healthcare settings or within 48 hours of admission; Hospital-acquired pneumonia (HAP) develops >=48 hours after admission; Ventilator-associated pneumonia (VAP) develops >=48 hours after intubation. HAP/VAP pathogens are typically more resistant (MRSA, Pseudomonas, Acinetobacter) requiring broader-spectrum empiric coverage.