West Nile virus, a mosquito-borne flavivirus, was initially isolated in Uganda in 1937. The first human cases of West Nile virus infection transmitted in the western hemisphere were recognized in New York City in late August 1999 in a cluster of encephalitis cases. A total of 59 hospitalized cases and seven deaths were recorded. Since that time the virus has been found in mosquitoes (the vector), birds, horses, humans, and other mammals.
