West Nile virus is a mosquito-borne disease that can cause encephalitis, or a brain infection.
Mosquitoes acquire the virus from birds and pass it on to other birds, animals and people.
In 1999, West Nile virus first appeared in New York. For the first time, in the summer of 2000, Pennsylvania found the virus in mosquitoes, birds and a horse. Mosquitoes spread this virus after they feed on infected birds and then bite people, other birds and animals. West Nile virus cases occur primarily in the late summer or early Fall, although the mosquito season is April through October. People can only become infected with West Nile virus after being bitten by an infected mosquito.