Cause

The most common way of acquiring the West Nile virus is from being bitten by an infected mosquito. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. The virus is located in the mosquito's salivary glands. When an infected mosquito bites a human or an animal to feed, the virus may be injected into the animal or human, where it may multiply, possibly causing illness.

You cannot get West Nile virus from touching or kissing a person who has the disease, from a health care worker who has treated someone with the disease or from handling a bird that has been infected with the disease. Additional routes of human infection became apparent during the 2002 West Nile epidemic. It is important to note that these other methods of transmission represent a very small proportion of cases. Investigations have identified virus transmission with transplanted organs and through blood transfusions. There is one reported case of a virus transmission from the mother to her unborn child through the placenta. There is also one reported case West Nile virus transmission through breast-milk.

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This article was reviewed and updated June 2007.