Virus, belonging to the family Flaviviridae, that can cause encephalitis (inflammation of the brain).
West Nile is predominantly a fatal infection of birds but can be transmitted to humans by mosquitoes. Most human infections are inapparent or mild, causing a flu-like illness that lasts only a few days. A minority develop encephalitis, characterized by headache, fever, neck stiffness, and muscle weakness, that has proved fatal in some cases. There is no cure. In severe cases, intensive medical care, involving intravenous fluids and respiratory support, is necessary. The virus was originally confined to Africa, the Middle East, and Southeast Asia but has spread to Europe and North America.