Mosquitoes are the primary vectors for the West Nile virus, and they contract the disease when they bite an animal that is infected.
Birds are the primary reservoirs, or carriers, of the West Nile virus and mosquitoes who bite these birds can then transfer the virus to another animal when they bite again.
The West Nile virus is passed from one animal to another through the transference of blood when a female mosquito bites multiple animals.