The virus is transmitted by mosquitoes.
A man in his 80s in Cook County became ill in mid-June, the Illinois Department of Public Health said in a news release Tuesday.
Last year, there were 42 human cases and four deaths reported in Illinois.
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"While we continue to battle the COVID-19 pandemic, we must also remember to take steps to protect our health from other illnesses," IDPH Director Dr. Ngozi Ezike said. "West Nile virus is something we see every year in Illinois and it is important people take steps to prevent mosquito bites and the viruses they carry by wearing insect repellent and getting rid of standing water around their homes."
Common symptoms 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 IDPH 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.
Additional information about West Nile virus can be found on the IDPH website.