West Nile Virus Home
> FAQ Index > Cases of West Nile
of West Nile Human Disease
How many cases of West Nile disease in humans have occurred in
A. Our Statistics,
Surveillance, and Control page contains maps showing the distribution
of West Nile virus-related human disease cases, by state, in the
No reliable estimates are available for the number of cases of West Nile encephalitis that occur worldwide.
What proportion of people with severe illness due to West Nile
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.
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 http://www.cdc.gov/ncidod/dvbid/westnile/surv&control.htm.