West Nile virus can infect humans, birds, mosquitoes, horses and some other mammals. It is part of a group of disease-bearing viruses called "flaviviruses," which cause yellow fever, dengue fever and Japanese encephalitis. West Nile virus is common in Africa, West Asia and the Middle East; however, in the summer of 1999, the eastern United States reported many cases of West Nile virus infection in humans. Since then, West Nile virus has continued to be reported in an increasing number of states, including California.

Most individuals have no symptoms from West Nile virus infection. Approximately 15 to 20 percent of individuals develop West Nile Fever, a mild illness characterized by body aches, eye pain, skin rash, lymph node swelling, nausea and/or vomiting. A small percentage of individuals have severe symptoms from West Nile virus, including meningitis, encephalitis and acute flaccid paralysis. The risks factors associated with severe symptoms have not been clearly defined but appear to include older age, high blood pressure, cardiac disease and diabetes.

The Palo Alto Medical Foundation regularly tracks reports on West Nile activity and has procedures to respond to the West Nile virus. The resources on this page have been chosen by PAMF to provide educational and preventive resources for our patients and the community, and will be updated as needed.


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