Transmission

Most people infected with West Nile Virus got it from the bite of an infected mosquito. A mosquito becomes infected when it feeds on the blood of a bird that is infected with the virus. About 2 weeks later, the mosquito becomes capable of passing the virus to people and animals by biting them.
Mosquito on leaf
In a very small number of cases, West Nile Virus also has been spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby.

There is no evidence to suggest that people can get WNV by touching or kissing someone who is infected, or from being around a health care worker who has treated an infected person. Likewise, there is no evidence to date that the virus can pass from infected animals (horses, pets, etc.) to people.
Transmission Cycle

Birds (Reservoirs)

Bird standing on wood post
West Nile Virus is first transmitted when a bird that's infected with the virus is bitten by a mosquito.

Mosquitoes (Vectors)

Mosquito biting skin

Humans (Incidental Infection)

Family walking in grass with picnic items
About 2 weeks later, the mosquito becomes capable of passing the virus to people and animals by biting them.

Horses (Incidental Infection)

Adult horse and 2 young horses running