Pertussis, one of the most communicable respiratory diseases, has a wide spectrum of severity, which generally decreases with age. Adults with waning immunity, who have subclinical pertussis, frequently infect nonimmunized or underimmunized children within the same household. High levels of pertussis activity persist, even in highly immunized populations. Infants less than 2 months of age have the highest attack rate and greatest morbidity and mortality. Serious complications include apnea, pneumonia, encephalopathy, and recurrence of coughing spasms with cyanosis. These complications further contribute to a protracted disease course in young infants. Although a decrease in the incidence of pertussis has occurred since the advent of active immunization in the United States, recent data show a striking resurgence in cases. Widespread transmission of disease, even in immunized individuals, and subclinical adult infection, which serves as a reservoir for disease in young infants, underscore the need for a more effective vaccine immunization strategy.