What is West Nile Virus? West Nile Virus (WNV) is transmitted by mosquitoes that feed off birds infected with the virus. After mosquitoes have been infected with WNV, they may then infect humans, birds, horses, and other animals by biting for blood. Although the majority of people and animals infected with WNV do not show any symptoms, it has the potential to cause mild to severe illness. The virus may cause illness known as West Nile encephalitis (inflammation of the brain).