virus belonging to the family Flaviviridae, related to viruses that can cause yellow fever and dengue and more closely to viruses that can cause encephalitis (inflammation of the brain). Predominantly an infection of birds, West Nile virus is highly fatal for many avian species (e.g., crows (crow) and other corvids (Corvidae)). A threat to human health occurs when infected birds (bird) are bitten by mosquitoes (mosquito), which then transmit the virus to humans. Most human infections are inapparent or mild, causing a flulike illness that usually lasts only a few days. However, in a minority of infected persons, particularly in those over the age of 50, the virus multiplies in the lymphoid tissue and circulates in the bloodstream (possibly also multiplying in leukocytes, or white blood cells) before reaching the brain, resulting in encephalitis. Symptoms of West Nile encephalitis include headache, fever, neck stiffness, disorientation, and muscle weakness. Death may result.

West Nile virus has been known and studied for decades, but historically it has been largely confined to Africa, the Middle East, and parts of Russia, India, and Indonesia, where it has caused occasional, usually minor, epidemics of denguelike illness or sporadic encephalitis. However, in recent years the virus has been imported more broadly into Europe by migratory birds. In 1999 West Nile virus was imported into the United States, spreading also into Canada and the Caribbean.

There is no specific treatment for infection with West Nile virus. In severe cases, intensive medical care is necessary, with continuous monitoring of respiratory function, management of fluid and electrolyte balance, and prevention of secondary infections.