West Nile encephalitis is an infection of the brain caused by West Nile virus, a close relative of St. Louis encephalitis virus.
People get West Nile encephalitis from the bite of a mosquito that is infected with West Nile virus.
West Nile virus is NOT transmitted from person to person. Natural Transmission Mosquitoes become infected with West Nile virus when they feed on infected birds that carry the virus in their blood. After 10 to 14 days, the mosquito's salivary glands become infected and those infected mosquitoes can then transmit West Nile virus to humans and other animals while biting them to take blood. During blood feeding, the mosquito injects the virus into the animal or human, where it multiplies and may cause illness. Most people infected with West Nile virus have no symptoms of illness, but some may become ill 3 to 15 days after the bite of an infected mosquito.