West Nile (WN) virus originated in remote areas of Africa, Asia, eastern Europe, and the Middle East. It is transmitted by mosquitoes. The virus was first detected in the United States in New York City in 1999. Usually people and animals that are infected with the virus show no symptoms or suffer only mild illness. In rare cases, the virus can cause a more serious condition called encephalitis, an inflammation of the brain, which can be fatal. Individuals over the age of 50 have the highest risk for encephalitis caused by WN virus.