West Nile virus is a mosquito-borne virus that can cause encephalitis (an inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord) in humans and other animals. The virus is named after the West Nile region of Uganda where it was first isolated in 1937. The virus appeared for the first time in the United States during a 1999 outbreak in New York that killed seven people. The first human case of the virus in Virginia appeared in 2002.
Anyone can get West Nile virus infection if bitten by an infected mosquito; however, even in areas where transmission of West Nile virus is known to be occurring only a small proportion of mosquitoes are likely to be infected (1 in 100). Even if a person is bitten by an infected mosquito, the chance of developing the illness is only 1 in 200. The elderly and persons who have weakened immune systems are at greater risk of developing a more severe form of the illness.
West Nile virus is spread to humans, birds, and other animals through the bite of an infected mosquito. A mosquito becomes infected by biting a bird that is carrying the virus. West Nile virus is not spread from person to person, but a small number of cases were a result of blood transfusions or organ transplants from infected people. There may also be a risk of infected mothers transmitting the virus to their unborn or nursing children.
In areas where West Nile virus has been detected, only a small proportion of mosquitoes are likely to be infected. Most people bitten by an infected mosquito do not become sick. Less than 1% of all people who are infected will become seriously ill.