West Nile virus: The mosquito-borne virus that causes West Nile fever. One of the flaviviruses, a family of viruses also responsible for dengue, yellow fever, and tick-borne encephalitis virus. Like the other flaviviruses, the West Nile virus is a positive-strand RNA virus containing three structural proteins and a host-derived lipid bilayer. The West Nile virus is an icosahedral structure of about 500 angstroms in diameter. The overall structure of the West Nile virus most closely resembles that of dengue fever virus but the surface proteins are slightly different, which may give West Nile virus some of its unique features.