SPRINGFIELD, Ill. (AP) — The Centers for Disease Control and Prevention has agreed to investigate a confirmed case of West Nile virus in Illinois.

The Illinois Department of Public Health says Wednesday the state saw its first human case of the virus for 2018 in mid-May. The case occurred earlier in the season than usual, prompting the department to call on the CDC for further testing.

The first confirmed case for 2017 occurred in mid-July. The state saw 90 human cases of the virus last year, including eight deaths.

West Nile is spread through the bite of an infected mosquito. Often there are no symptoms, but those older than 60 are more likely to die or to develop a severe illness.

The Department of Public Health suggests people to stay alert and use insect repellent when outside.