Flu cases are showing up in all areas of the state.
Illinois Public Health Director Lamar Hasbrouck said flu activity is happening earlier this year than most. The virus typically peaks in Illinois in January and February.
Dr. Hasbrouck said some schools have had high absentee rates because of students being sick.
"So if you have mild to moderate symptoms, you should stay at home and nurse yourself. Stay away from work. Stay away from school and get better,” said Hasbrouck.
“But if you're having severe symptoms, by all means reach out to your health care provider.”
Hasbrouck said severe symptoms would include difficulty breathing and those with a chronic condition.
The state tracks the flu's prevalence through the number of people being admitted to hospitals. But he said since most cases are treated at home, many more people are dealing with it than what the state's numbers show.
The flu vaccine is less effective this year because one of the strains it targets has mutated. That means it offers no protection for that specific strain.