This year, a particularly potent B strain of the virus seems to be causing many of the illnesses.
However, influenza A virus also appears to be increasing.
According to the Centers for Disease Control and Prevention, there have been about 9.7 million flu cases across the United States so far this season, with 87,000 hospitalizations and 4,800 deaths.
Samantha Franco is seeing firsthand just how dangerous the flu can be to people who were otherwise healthy.
Her daughter Jayden Carrillo is now fighting for her life after coming down with the flu.
Jayden is an athlete and was always healthy, her mom says. For the past eight days she's been hospitalized for what started out seeming like a simple cold. She did not receive a flu shot.
"I used to think people get the flu, you have a fever for a few days, a cough, congestion and you get better," Franco said. "Now i see the severity and I'm like wow this is serious, this flu virus is serious and it can be deadly."
She was diagnosed with influenza B and then pneumonia and developed a bacterial infection in her blood. She was put on life support on Friday.
Dr. Bernard Camins, medical director for infection prevention at Mt. Sinai hospital, said the flu can often lead to secondary complications.
"Someone who may have been sick with the flu and then recovered but then is sick again, that's a sign potentially that they may have bacterial pneumonia as a complication," he said.
Franco is sharing her daughter's story in the hope it can help others and encourage people to get flu shots.
"I didn't think that this would happen to my child. You hear of these stories, but you think that can't happen to me. My kid's healthy. But it did."
The good news, however, is that Jayden does appear to be showing some signs of improvement and doctors hope she will recover enough to no longer require life support within a few days.
The CDC says it is seeing elevated levels of outpatient flu cases but that hospitalization rates and deaths related to pneumonia and influenza remain relatively low.
The early dominance of the B-strain flu virus seems to hit children the hardest, with 32 pediatric deaths so far this season - the most this early in the season since the CDC started keeping track 16 years ago.