Illinois joins 24 other states in reporting the highest levels of flu.
Seven deaths have been reported in the state, including six in Cook County.
Statewide, another 122 patients have been treated in hospital intensive care unites.
Health professionals say the H1N1 flu tends to target young people and middle-aged adults more than senior citizens.
"We are seeing people who are generally healthy in their 30s, 40s and 50s come down with very serious flu-related illness, and in some cases need life support," said Dr. Matthew Davis of the University of Michigan Health System.
The H1N1 strain is the same one that caused a pandemic five years ago.
Most cases typically occur in January and February,
"We may see an increase within a week because people are just being exposed to it more," Allina West Health Dr. John Svenningsen said.
Officials say the best way to protect yourself is to get a flu shot which will help weaken symptoms if you get sick.