Biologists once thought that West Nile Virus could only be transmitted through bites by mosquitoes like this treehole mosquito, but now believe the disease may also spread directly from one bird to another

Culex pipiens, an enzootic vector of West Nile virus

West Nile Virus is generally carried by mosquitoes that infect birds; however, the virus can be spread to humans. The Asian Tiger Mosquito is a carrier of the virus and is described as an "aggressive day biter".