West Nile Virus

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West Nile Virus (WNV) is a mosquito-borne flavivirus that is widely distributed in Africa, West Asia, and the Middle East. WNV was first detected in the Western Hemisphere in 1999 during an outbreak of encephalitis in New York City, and has since spread across the United States. 

WNV is spread by the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds. Infected mosquitos can then spread the virus to humans and other animals when they bite. WNV is not communicable from person to person, with the rare exceptions of blood transfusions, organ transplants, breastfeeding, or perinatal transmission.

The incubation period, the time from exposure to the time symptoms start, for WNV infection is thought to range from about 2 to 14 days, although longer incubation periods have been documented in immunosuppressed persons.

Most persons who become infected with WNV develop no clinical illness or symptoms. Of the approximately 20% of infected people who do develop symptoms, most develop what has been termed West Nile fever (WNF), characterized by fever, headaches, and body aches. 

Approximately 1 in 150 persons infected with WNV will develop severe illness, called West Nile neuroinvasive disease (WNND) because it affects a person's nervous system. Specific types of WNND include: West Nile encephalitis, West Nile meningitis, West Nile meningoencephalitis and West Nile poliomyelitis. Symptoms may include high fever, headache, stiff neck, disorientation, coma, convulsions, and paralysis.

The documents and websites below may answer more of your questions about West Nile Virus and how to protect yourself.    

Information for the Public
Information for Health Facilities and Health Professionals
Information for Specific Groups
  • West Nile Virus Information for Seniors  English  Spanish
    California Department of Public Health (CDPH)
Other Language Materials
Additional Resources

For more information, contact the Epidemiology Program 619-692-8499 or send us an e-mail.