West nile virus infection in the United States: overview as a public health issue.

In August 1999, West Nile virus (WNV), a mosquito-transmitted flavivirus, appeared in New York City. This represented the first time a major outbreak of this Old World virus caused an epidemic in the western hemisphere. By December 2002, the outbreak had spread, probably via avian migratory flyways, to involve 44 states and the District of Columbia. The future epidemiology of WNV infection in the United States will be difficult to forecast. In the absence of an effective human vaccine, the only means of prevention and control is reducing contact between humans and infected mosquitoes. An effective public health infrastructure will be critical in monitoring the progress of this epidemic and developing a strategy to control it.