West Nile Virus is a virus which was previously associated and commonly found in Africa, West Asia and the Middle East. It is not known how long it has been in the United States, but the Centers for Disease Control and Prevention (CDC) believe the virus probably has been in the US since early summer of 1999. It is closely related to the St. Louis encephalitis virus found in the US. The virus can infect humans, birds, mosquitoes, horses and other animals.