West Nile virus(WNV) was first isolated from a febrile1 adult woman in the West Nile District of Uganda in 1937. The virus became recognized as a cause of severe human meningoencephalitis2 in elderly patients during an outbreak in Israel in 1957.

　　WNV has been described in Africa, Europe, the Middle East, west and central Asia, Oceania, and most recently, North America. The first appearance of WNV in North America in 1999, with encephalitis3 reported in humans and horses, and the subsequent spread in the United States may be an important milestone in the evolving history of this virus. Experts believe WNV is established as a seasonal epidemic in North America that flares up4 in the summer and continues into the fall.

　　WNV affects the central nervous system. The most serious manifestation5 of WNV infection is fatal encephalitis. Its symptoms vary. Approximately 80 percent of people who are infected with WNV will not show any symptoms at all. Up to 20 percent of the people who become infected will display mild symptoms, including fever, headache, and body aches, nausea6, vomiting, and sometimes swollen lymph glands7 or a skin rash on the chest, stomach and back. Symptoms typically last a few days. About one in 150 people infected with WNV will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor8, disorientation9, coma10, tremors11, convulsions12, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological1 effects may be permanent.

　　Generally, WNV is spread by the bite of an infected mosquito. People typically develop symptoms between 3 and 14 days after they are bitten by the infected mosquito. There is no specific treatment for WNV infection.