At least 25 schools in the area are reporting "clusters" of students coming down with a highly-contagious norovirus.
Officials aren't identifying which schools are affected, but said they include elementary, middle and high schools, as well as one university.
Norovirus is highly contagious and can survive on surfaces and food for up to 12 days.
"If someone has a norovirus infection and who may not have the optimal hygiene can actually contaminate food and that can go on and create illnesses in other people who eat the food," said Dr. Susan Gerber with the Cook County Department of Public Health.
The reports started coming in at the end of October, officials said. The main symptoms of the norovirus include diarrhea, vomitting and nausea. The symptoms last one to three days.
Parents are urged to keep sick kids home for 24 hours after symptoms have cleared.
The health department is asking schools to report clusters of two or more students or staff with gastrointestinal illness to contact them at 708-492-2150.