West Nile (WNV) is spread by the bite of an infected mosquito, most symptoms are mild, but there have been cases of encephalitis (inflammation of the brain), meningitis (inflammation of the lining of the brain and spinal cord), or polio-like paralysis.
It was first identified in the United States in New York in 1999. Mosquitoes become infected by biting a bird that carries the virus. The genus Culex is associated with the spread of west Nile and St. Louis encephalitis. There are over 30 species nation-wide known to be vectors for West Nile and St. Louis encephalitis.