The involvement and expertise of infectious disease physicians, microbiologists, and public health practitioners are essential to the early detection and management of epidemics--both those that are naturally occurring, such as the 1999 outbreak of West Nile virus (WN virus) in New York City, and those that might follow covert acts of bioterrorism. The experience with the WN virus outbreak offers practical lessons in outbreak detection, laboratory diagnosis, investigation, and response that might usefully influence planning for future infectious disease outbreaks. Many of the strategies used to detect and respond to the WN virus outbreak resemble those that would be required to confront other serious infectious disease threats, such as pandemic influenza or bioterrorism. We provide an overview of the critical elements needed to manage a large-scale, fast-moving infectious disease outbreak, and we suggest ways that the existing public health capacity might be strengthened to ensure an effective response to both natural and intentional disease outbreaks.