West Nile Virus

West Nile virus is maintained in nature through a cycle between mosquitoes and birds, with humans and horses as incidental hosts who do not usually pass the virus onward. It has become established across much of the world, including large parts of the United States, Europe, and the Middle East, with case numbers rising in summer and early fall mosquito seasons. Most infections cause no symptoms at all, so the true burden is likely higher than reported case counts suggest. Virus Tracker surfaces regional activity where open data mirrors seasonal surveillance reports.

The virus is transmitted by the bite of an infected Culex mosquito, which becomes infected after feeding on a bird carrying the virus. Rare transmission has occurred through blood transfusion, organ transplant, and from mother to child, but mosquito bites remain the overwhelming source of infection.

About 80% of infected people have no symptoms. Roughly 20% develop fever, headache, body aches, and sometimes a rash, usually resolving within about a week. Less than 1% develop severe neurological illness, such as encephalitis or meningitis, which is more common in older adults and people with weakened immune systems.

There is no vaccine for humans, so prevention relies on avoiding mosquito bites through repellent use, protective clothing, and eliminating standing water where mosquitoes breed. Community mosquito control programs help reduce vector populations during peak transmission season.

Surveillance often combines human case reporting with mosquito trapping and testing, and monitoring of bird die-offs, which can serve as an early warning sign of virus activity in an area before human cases appear.

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