Hospitals across the country are battling one of the worst outbreaks of the flu in a decade.

Influenza has already claimed the lives of 18 children and infected thousands of people in 41 states.

In Illinois, doctors say they haven't seen this many cases since the swine flu outbreak in 2009.

"It's epidemic. It's been huge and there's been a lot of admissions for flu more than ever," Dr. David Rosenberg, a pediatric pulmonologist, said in an interview with ABC News.

More than 860 people have been hospitalized in Ohio for flu symptoms. That is a significant increase above the 65 hospitalizations the Buckeye Sate saw this time last year with people feeling flu-like symptoms.

"Our normal volume here would be about about 40 patients. We're seeing 75 to 80 patients a day now," Neal Smith, an Ohio doctor, said.

There are still five weeks remaining until the end of flu season. Emergency rooms around the country are expecting more activity.