Yersinia pestis, the etiologic agent of plague, is endemic in the western United States; 105 cases were reported between 1970 and 1979. Plague may manifest in one of three clinical forms: bubonic, septicemic, or pneumonic. Bubonic and septicemic plague represent relatively little risk for human-to-human transmission to contacts, although heavily infected secretions, such as drainage from a bubo, pose a theoretical risk, especially if they are aerosolized from a syringe during diagnostic aspirations. Pneumonic plague may be highly contagious to contacts and poses a greater risk. The Plague Branch, Center for Disease Control recommends that all patients with plague be placed in strict isolation for the first 48 hours of treatment because of the possibility that pneumonia may supervene. If it does not, wound and skin precautions are adequate for the duration of hospitalization. Untreated plague pneumonia is an epidemiologic emergency. All contacts must be identified promptly and those with face-to-face exposure should receive abortive antibiotic therapy. All contacts should be under surveillance, with twice-daily temperature checks, for seven days.