Ventilator-associated pneumonia (VAP) develops >= 48 hours after endotracheal intubation, with an incidence of 5-15% of mechanically ventilated patients. Pathogenesis involves aspiration of colonized oropharyngeal or gastric secretions around the ETT cuff into the lower airways. The ETT bypasses upper airway defenses (glottis, cough reflex) and creates a conduit for bacterial migration via biofilm formation on the inner tube surface. Early-onset VAP (< 5 days) is typically caused by community organisms (S. pneumoniae, H. influenzae, MSSA), while late-onset VAP (>= 5 days) involves MDR organisms: MRSA, Pseudomonas aeruginosa, Acinetobacter, Klebsiella (ESBL-producing), Stenotrophomonas. Risk factors include reintubation, supine position, sedation depth, gastric acid suppression, and transport out of ICU. Prevention bundles have reduced VAP incidence by 40-70%.