It was first identified in the United States in the late summer of 1999.
People become infected with West Nile virus from the bite of an infected mosquito.
Mosquitoes become infected by biting birds which are infected with the virus. Some birds may die of the disease, but they are not known to transmit the disease to humans. Not all humans infected with the disease will become ill. Most infections are mild and symptoms may include fever, headache, body aches, skin rashes and swollen lymph glands. Most people recover from the disease.