Pertussis is a highly contagious respiratory infection caused by the gram-negative bacterium Bordetella pertussis. The bacteria attach to ciliated respiratory epithelial cells using adhesins (filamentous hemagglutinin, pertactin) and release toxins — pertussis toxin (PT) disrupts cellular signaling causing lymphocytosis and immune dysregulation, while tracheal cytotoxin destroys ciliated cells, impairing the mucociliary escalator. The disease progresses through three stages: catarrhal stage (1-2 weeks: cold-like symptoms, most contagious), paroxysmal stage (1-6 weeks: severe coughing fits with inspiratory whoop, post-tussive vomiting), and convalescent stage (weeks to months: gradual cough resolution). Infants under 6 months are at highest risk for complications including apnea, pneumonia, seizures, and death.