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West Nile Virus


West Nile Virus PDF Print E-mail

What Is West Nile disease?

Viruses and bacteria can cause encephalitis (an inflammation of the brain) in humans and other animals. West Nile encephalitis is a mosquito-borne infection of the brain caused by West Nile virus, a close relative of St. Louis encephalitis virus. It is thought to have been introduced into the New York City area in 1999 and by 2003 had spread across the country to California. West Nile virus is commonly found in Africa, West Asia and the Middle East. Although it is not known how the virus was introduced to the U.S., it may have entered this country in an infected traveler, bird or mosquito.

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