Outbreaks can occur, causing the number of cases to be much higher, but these are uncommon in the U.S. But in 2002, an outbreak of mosquito-borne West Nile virus in the U.S. caused numerous infections and deaths. Still, far more people who have West Nile virus develop mild or no symptoms rather than encephalitis.
The number of cases caused by mosquito-borne viruses varies by location throughout the world. In the U.S., mosquito-borne encephalitis (such as St. Louis encephalitis, La Crosse encephalitis, and West Nile encephalitis) is rare. Tick-borne viral encephalitis is even rarer.
Infection with the rabies virus can cause encephalitis and is almost always fatal if it is not treated before symptoms develop.
People who have impaired immune systems, especially those with AIDS (acquired immunodeficiency syndrome), are at increased risk of developing encephalitis from cytomegalovirus, toxoplasmosis, and other infections.
Last Revised: October 26, 2011
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