The epidemiology of pertussis has changed in recent years. First, pertussis in adults is far more common than previously thought. Second, in many instances, the disease in adults is atypical or asymptomatic. Third, adult pertussis occurs despite a prior history of full immunization and, indeed, in persons with a prior history of natural disease. Large outbreaks of pertussis have occurred in healthcare facilities through failure to recognize and isolate infected infants and children, failure to recognize and treat disease in staff members, and failure to institute control measures rapidly. Appropriate use of work restriction and erythromycin prophylaxis may decrease the likelihood of institutional outbreaks.