It is a mosquito-borne virus that can cause minor illness, encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord). The virus is spread by mosquitoes after they feed on infected birds and then bite people.
It is not spread by person-to-person contact and there is no evidence that people can get the disease by handling infected animals. It can occasionally result in serious illness or death, however, most persons infected show no symptoms.
West Nile Virus was first recognized in the United States in 1999. It is not known how the virus was first introduced into the United States, but since the initial appearance it has spread rapidly and can now be found throughout the United States. However, the heaviest concentrations have been in the eastern half of the country (east of the Mississippi River). West Nile Virus occurs primarily in the late summer or early fall (August - September), although the peak season is usually from April to October.