West Nile fever is a mosquito-borne febrile illness seen in Africa, Asia, and Europe, but reported in North America only once. West Nile virus is of the family Flaviviridae, genus Flavivirus. Several West Nile fever outbreaks were reported from Israel in the 1950s and in 1980. Since the mid-1990s, frequency and apparent clinical severity of West Nile virus outbreaks have increased. Outbreaks in Rumania (1996), Russia (1999), and Israel (2000) involved hundreds of persons with severe neurologic disease. In the summer of 1999, West Nile virus was recognized in the among Western Hemisphere for the first time when it caused an epidemic of encephalitis and aseptic meningitis among residents of greater New York City. Other outbreak occurred during the summer of 2000. West Nile fever in humans usually is a febrile, influenza-like illness characterized by abrupt onset. Occasionally (less than 15% of cases) are acute aseptic meningitis or encephalitis. Although human vaccines for West Nile virus are under development, the most effective to way reduce local population of mosquitoes is to eliminate sources of standing water and preventing vector mosquitoes from biting humans by using mosquito repellents and barrier methods.