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West Nile Virus Causes

West Nile virus is transmitted to humans by the bite of an infected mosquito. Mosquitoes become infected by biting birds that harbor the virus. The virus is not spread from person to person or from infected birds to humans without a mosquito bite. The virus has now been found in 111 bird species and about a dozen mammals.

• How West Nile virus entered New York is not entirely clear. The most likely explanation is that the virus was introduced by an imported infected bird or by an infected human returning from a country where West Nile virus is common. Before the 1999 New York outbreak, West Nile encephalitis had been identified only in Africa, Asia, the Middle East, and only rarely in Europe.

• Most cases of West Nile occur during the warm weather months. Nonetheless, the mild climate in southern states is expected to sustain the mosquitoes beyond those months.