According to the CDC, hospitalizations of children under five are the highest among all the age groups right now.
The good news is that compared to last year's season - which doctors say was one of the worst in nearly a decade - this one started late, so there's still time to prevent illness.
Doctors and pharmacists nationwide are trying to get the word out that it's better late, than never. The late start to the 2018-2019 flu season means there are still several more months people will be at risk.
WATCH: What you should know about the flu shot
"It does take two weeks to become fully effective in your body. Next year, get it a little bit earlier. It's never too late to get the flu shot, but the earlier you get it, the more coverage you've got," said Walgreens pharmacy manager Danielle Soriano.
According to the CDC's latest figures, widespread flu activity is now being reported in 19 states, including Illinois. Children are the hardest hit, and 13 have already died nationwide.
WATCH: How to keep your kids safe during flu season
"H1NI tends to affect children more than it does adults and, in fact, the hospitalizations that we've seen this year to date there are more children that are being hospitalized than adults," said Dr. Sharon Welbel, an epidemiologist at Cook County's Stroger Hospital.
Dr. Welbel said 37 people have been hospitalized there this season. Only five of them had been vaccinated.
"When people get vaccinated, they're less likely to get hospitalized, they're less likely to go into the intensive care unit," Dr. Welbel said.
It's a message DePaul student Madi Ellerbrock is taking to heart.
"All of a sudden I was like, I've got to get the shot 'cause I got the flu last year and it was brutal so I didn't want to do that again," Ellerbrock said.
Doctors say it's important to monitor yourself and your children especially for signs of the flu. If you think you have it, there is medication that can be taken for it, as long as it's diagnosed early, so those who are ill should not delay in getting medical attention.