The first cases of West Nile virus were identified in 1999 in the New York City area. West Nile virus is primarily a disease of birds found in Africa, West Asia, and the Middle East. It is transmitted to humans mainly via mosquito bites (mainly from the species Culex pipiens). The female mosquito catches the virus when it bites an infected bird and then passes it on when it later bites a human. In humans it causes encephalitis, an infection of the brain that can be lethal.