West Nile virus (WNV) is a member of the family Flaviviridae. Transmission predominantly involves mosquitoes of the Culex genus and wild birds, and humans are incidental hosts. Human infections in areas of endemicity are usually mild or subclinical. Severe disease is commonly associated with the elderly. West Nile virus was first isolated from the blood of a febrile woman in the West Nile district Uganda in 1937 and was subsequently isolated from patients, birds, and mosquitoes in Egypt in the early 1950s. Outbreaks in humans have occurred in at least 17 countries in Africa and Europe. An outbreak of viral encephalitis in New York City in 1999 that involved 62 people and resulted in seven deaths was the first recognized virus in North America.