West Nile Virus | Causes & Risk Factors


How is West Nile virus spread?

West Nile virus is most often spread by mosquitoes. Mosquitoes become infected by biting birds that carry the virus. People can get West Nile virus when an infected mosquito bites them. This happens most often in the warm-weather months of spring, summer and early fall. You cannot get West Nile virus from another person or from your pet.

During the 2002 West Nile virus epidemic in the United States, a few people got the virus through blood transfusions and organ transplants. However, the risk of getting the virus in these ways is very low.

A few cases were also reported of the West Nile virus being passed from a pregnant or breastfeeding woman to her baby. However, these cases are extremely rare.

Who is at risk for infection with West Nile virus?

People who live where West Nile virus has been found in humans, birds, horses or mosquitoes are at risk for infection. You are also at a greater risk if you spend lots of time outdoors during the warmer months or if you don't protect your skin with an insect repellent that contains DEET. However, even in areas where the virus has been reported, it's very unlikely that a person will get sick from a mosquito bite.

People 50 years of age and older and people who have weakened immune systems are at greatest risk of becoming severely ill from West Nile virus. Remember, less than 1% of the people who do get infected with West Nile become severely ill.

How many people with severe West Nile virus infection die?

Most people who are infected with West Nile either do not develop symptoms or only develop the mild symptoms of West Nile fever. Less than 1% of people infected with the virus will develop severe symptoms or complications, and almost all of those people fully recover. Of the small number of people who do get severely ill from West Nile virus, about 10% will die.


West Nile Virus in the United States: An Update on an Emerging Infectious Disease by GD. Huhn, M.D., M.P.H.T.M., JJ Sejvar, M.D., SP Montgomery, D.V.M., M.P.H., and MS Dworkin, M.D., M.P.H.T.M. (American Family Physician August 15, 2003, http://www.aafp.org/afp/20030815/653.html)

Written by familydoctor.org editorial staff

Reviewed/Updated: 04/14
Created: 04/04