The disease caused by the virus is known as encephalitis and can be severe in the elderly, but it is usually mild in healthy adults and children. Humans infected with WN virus can experience mild to severe symptoms including elevated fever that comes on quickly and lasts from 5 to 6 days. Other symptoms include severe headache, a rash, swollen lymph nodes, gastrointestinal problems, and pain associated with the eyes, muscles, and back. In serious cases, the symptoms of encephalitis can cause death. In domestic animals, some of the signs of disease have been observed in horses, but most of the infections have only resulted in a low grade fever.
Mosquitoes obtain the virus from feeding on infected birds.
The virus is then passed along to humans when an infected female mosquito takes a second blood meal from a human instead of a bird. As they are feeding on human blood, they release saliva that contains the virus. The saliva then enters the human bloodstream carrying the virus with it. The mosquito species responsible for the 1999 NYC outbreak have not yet been identified with certainty. The most likely candidates, however, are members of the Culex pipiens species complex.