The West Nile Virus generally infects its host through mosquitoes. However, there are also numerous cases of it spreading on farms through cattle.
The reason mosquitoes are the best method of transportation of the West Nile Virus is because the mosquitoes that carry the virus are completely immune to it, so it just passes from one person to another by a mosquito. They also cover a large distance in a short period of time.
The West Nile Virus has also been known to kill birds, but it's a lot rarer.