An assessment of the threat of bioterrorism suggests that a state-sponsor might provide terrorists with the necessary know-how, seed cultures, and specialized dissemination equipment. Alternatively, a wealthy terrorist organization might recruit scientists and engineers formerly employed by a state biowarfare program. A bioterrorist attack would probably involve the covert release of a microbial pathogen that would produce detectable illness only after an asymptomatic delay or incubation period when the microorganism is multiplying in the host to cause disease. In parallel with the emerging threat of bioterrorism, the United States faces a growing problem of infectious disease from natural sources. Over the past two decades, several well-known diseases have re-emerged in more virulent or drug-resistant forms or have spread geographically. At the same time, scientists have identified a host of previously unknown infections. This paper identifies the factors that have contributed to the problem of emerging infections. A major epidemic from a natural emerging infection or an act of bioterrorism would pose serious challenges to the U.S. public health system in four areas: recognition and diagnosis by primary health care practitioners; communication of surveillance information to public health authorities; epidemiological analysis of the raw surveillance data; and delivery of the appropriate medical treatment and public health measures. Policy recommendations offered in this paper pertain to bridging the gap between primary care providers and public health departments, between the human and animal health communities, and between public health specialists and intelligence analysts. 12 references