West Nile virus (WNV) is a mosquito-borne virus that can cause mild illness (West Nile fever) or severe symptoms (encephalitis or meningitis-- an inflammation of the brain) in humans and other animals. People primarily get West Nile from the bite of a mosquito that is infected with West Nile virus. Mosquitoes become infected with West Nile virus when they feed on infected birds that carry the virus in their blood. Although most people infected with West Nile virus recover completely, the virus can cause long-term health problems and, in some instances, can be fatal.