Mosquitoes carrying West Nile virus have been confirmed in Chicago, the city’s health department announced Friday.

The state health department announced mosquitoes carrying the virus were present in the state around a month ago, but outside the Chicago area. No human cases have been reported in the state this year.

The Chicago Department of Public Health uses more than 80,000 catch basins with larvacide to prevent and control the virus by monitoring infections and killing mosquitoes in certain areas if necessary.

“One of the best ways for Chicagoans to have a safe and healthy summer is by protecting themselves from mosquito bites,” said CDPH Medical Director Dr. Janna Kerins.

Wearing insect repellant and long sleeves can help prevent mosquito bites that could carry the virus, according to the health department. Using air conditioning and making sure windows and screens don’t have holes can keep them out of buildings.

Most mosquitoes don’t carry the virus, but mosquito bites are the most common way the virus transmits to humans.

Around 1 in 5 humans infected with the virus develop a fever and flu-like symptoms, and severe illness is possible, especially among people over 55 or with weakened immune systems.