West Nile Virus (WNV) is a mosquito-borne formerly old-world Flavivirus with a rapidly expanding geographic distribution, first isolated in the West Nile district of Uganda in 1937. Up until the appearance of WNV in the New York City area in 1999, the virus was found only in the Eastern Hemisphere, specifically in Asia, Africa, the Middle East, and Europe. In 1999 this zoonosis emerged in the New York City area, first recognized as a cause of mortality in several bird species, then as a cause of illness and mortality in humans. Since that time, WNV has firmly established itself in a large portion of the United States, and was the cause of the largest human West Nile epidemic and animal epizootic ever reported in 2002 (4,156 human cases of WN disease, with 284 deaths, 16,741 dead birds, 6,604 infected mosquito pools, and 14,571 equine cases). However, cases in the U.S. in 2003 eclipsed those reported in 2002 by greater than 100 percent: in 2003, 9858 human cases and 258 deaths were reported to CDC. Cases have also been documented in Canada, the Caribbean, and in Mexico. Human disease has been acquired in every state in the contiguous United States except Oregon and Washington, and is expected to occur in every state in 2004. The lone case in 2004 thus far has been reported in New Mexico. In addition to mosquito-borne disease, blood transfusion, transplacental transmission, organ transplantation, and breast-feeding associated cases have all been documented.

Migratory birds are thought to be the major introductory hosts of West Nile virus to new geographic areas. This virus causes subclinical illness in a large proportion of those infected, a mild febrile illness in 20%, and severe neurological illness (encephalitis, meningitis, or both) in 1 in 150 patients. Older patients are at greatest risk for severe illness, which can cause permanent neurological sequelae or death. There is currently no specific treatment for WNV infection, and prevention is based on limiting mosquito exposure in enzootic and epidemic areas.