Skip Navigation
Leading research to understand, treat, and prevent infectious, immunologic, and allergic diseases
Skip Content Marketing
  • Share this:
  • submit to facebook
  • Tweet it
  • submit to reddit
  • submit to StumbleUpon
  • submit to Google +

What Is West Nile Virus?

West Nile virus (WNV) is an emerging infectious disease that made its first appearance in the United States in 1999. The microbe that causes the infection belongs to a group of disease-causing viruses known as flaviviruses that are usually spread by ticks or mosquitoes. Other well-known diseases caused by flaviviruses include yellow fever, Japanese encephalitis, dengue, and Saint Louis encephalitis.


WNV was first isolated in Uganda in 1937. Today it is most commonly found in Africa, West Asia, Europe, and the Middle East. In 1999, it was found in the Western Hemisphere for the first time in the New York City area. In early spring 2000, it appeared again in birds and mosquitoes and then spread to other parts of the eastern United States. By 2004, the virus had been found in birds and mosquitoes in every state except Alaska and Hawaii.

More than 1100 cases of West Nile virus have been reported in the United States in 2012. This is the highest number of cases reported in the United States since the disease was first detected in the country in 1999. The majority of the cases have been reported from 5 states (Texas, Mississippi, Louisiana, South Dakota, and Oklahoma) and almost half of all cases have been reported from Texas, according to the Centers for Disease Control and Prevention.

Visit the CDC Web site for the most recent numbers of reported human cases of WNV infection in the United States.

Last Updated August 16, 2012

Last Reviewed August 16, 2012