West Nile virus, microorganism and the infection resulting from it, which typically produces no symptoms or a flulike condition. The virus is a flavivirus and is related to a number of viruses that cause encephalitis. It usually is transmitted through the bite of several mosquito species, and can infect humans and more than 200 animal species, including alligators, horses, and many common birds. A number of North American bird species, including the blue jay, crow, and house sparrow, act as reservoirs of the virus.

About one fifth of humans infected with the virus develop West Nile fever, which in most people is characterized by fever, headache, muscle ache, joint pain, nausea and vomiting, and in some cases rash and swollen lymph nodes (swollen glands). Less than 1% of all persons infected may develop a severe case, progressing to encephalitis encephalitis (ĕnsĕf'əlī`təs)
..... Click the link for more information. or meningitis meningitis (mĕnĭnjī`tĭs) or cerebrospinal meningitis
..... Click the link for more information. , about a week after the initial symptoms of West Nile fever appear. Loss of vision, tremors and convulsions, paralysis, coma, and, in some cases, death may result. Older persons and persons with a weakened immune system are most susceptible to severe cases of the disease, and they may suffer from longer-term effects including weakness and fatigue, headaches, memory loss and confusion, and depression. There is no treatment or vaccine for the virus except to alleviate the symptoms; controlling the mosquitoes that carry the disease is the most effective way to limit its spread.

West Nile virus was first identified in 1937 in the West Nile district of Uganda. It was subsequently found in much of the rest of Africa, the Middle East, and warmer regions of Asia and Europe. Its first recorded appearance in the United States was in Queens, N.Y., in 1999, and it since has spread to most of the United States and neighboring areas in North America.

West Nile virus

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.