The first virus-positive mosquitoes of the year were found in traps in Evanston, Kenilworth and Northfield.
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They were collected last week, and confirmed positive Tuesday.
Virus-positive mosquitoes have also been found in Skokie.
RELATED: West Nile virus 2021: Cook County man 1st human case of mosquito-borne disease in IL this year
To protect yourself, use insect repellent, wear loose-fitting clothes and avoid peak mosquito feeding times -- dawn and dusk.
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Also get rid of items around your property that can hold water where mosquitoes breed.
Common symptoms of West Nile virus include fever, nausea, headache and muscle aches. Symptoms may last from a few days to a few weeks. However, four out of five people infected with West Nile virus will not show any symptoms, the Illinois Department of Health has said.
In rare cases, severe illness including meningitis, or even death, can occur. People older than 60 and individuals with weakened immune systems are at higher risk for severe illness from West Nile virus.