Since the 1930s, West Nile virus (WNV) has been commonly reported to cause infection and fevers in humans in Africa, West Asia and the Middle East. Until 1999, human and animal infections had never been documented in the Western Hemisphere. In 1999 and 2000, outbreaks of WNV encephalitis (inflammation of the brain) were reported in persons living in the New York City metropolitan area.
In 1999 and 2000, 83 human cases of West Nile virus were reported; 9 died. In 2001, the first cases of WNV encephalitis outside of the New York City metro area occurred when 2 cases were reported from a rural Florida county in July and August. The virus has continually spread westward, and Colorado had a large amount of cases in 2003. In 2004, there were 830 human infections in California, including 2 in San Diego County. Neither of the two San Diego County cases proved to be locally acquired. WNV is transmitted to humans through mosquito bites. Mosquitoes become infected when they feed on infected birds that have high levels of WNV in their blood. Infected mosquitoes can then transmit WNV when they feed on humans or other animals. The disease is not spread from person to person.