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> FAQ Index > Cases of West Nile Encephalitis

Questions and Answers

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Cases of West Nile Human Disease

Q. How many cases of West Nile disease in humans have occurred in the U.S.?
A. Our Statistics, Surveillance, and Control page contains maps showing the distribution of West Nile virus-related human disease cases, by state, in the United States.

No reliable estimates are available for the number of cases of West Nile encephalitis that occur worldwide.

Q. What proportion of people with severe illness due to West Nile virus die?
A. Among those with severe illness due to West Nile virus, case-fatality rates range from 3% to 15% and are highest among the elderly. Less than 1% of people who become infected with West Nile virus will develop severe illness -- most people who get infected do not develop any disease at all.

Q. How can a person test positive for WNV infection at a blood bank, but not be considered a "case" by CDC?
A. A WNV "case" is a person who has become ill and been confirmed to have WNV infection. This infection might be either West Nile Fever, a mild illness with fever, or West Nile encephalitis or meningitis, more severe illnesses. Blood donors who do not become ill and do not develop symptoms are counted in a separate category because they are not considered "cases."

For more information on human cases, disease surveillance, and a map of cases, go to


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