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The first step in the transmission cycle of West Nile virus (WNV) happens when a mosquito bites an infected bird or animal and gets the virus while feeding on the animal's blood. The infected mosquito can then transmit the virus to another bird or animal when it feeds again.

Crows are highly susceptible to lethal infection, as are robins, blue jays, and other birds. Scientists have identified more than 138 bird species that can be infected and more than 43 mosquito species that can transmit WNV.

Although the virus usually cycles between mosquitoes and birds, infected female mosquitoes also can transmit WNV through their bites to humans and other "incidental hosts," such as horses. With so many susceptible hosts to amplify the virus and so many types of mosquitoes to transmit it, WNV has spread rapidly across the United States.

Most cases of human disease occur in elderly people and in people with impaired immune systems. In a very small number of cases, WNV has been spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby, according to the CDC.

Last Updated August 16, 2012

Last Reviewed August 16, 2012