The Centers for Disease Control and Prevention declared this flu season an epidemic, noting that 15 children have already died due to complications of the virus. 

Most of the flu cases this year - about 90 percent - were the H3N2 subtype. H3N2, also known as Influenza A, is normally circulated in pigs but can also infect people, according to the CDC. When a person is infected with the virus it's referred to as a "variant" virus. 

Anyone can become infected with the flu, but those under the age of 5, over the age of 65, pregnant women and those with certain long-term health conditions are at the highest risk. 

Fifteen children already died this season due to the flu in nine different states: Arizona (one), Colorado (one), Florida (two), Minnesota (two), North Carolina (two), Nevada (one), Ohio (two), Texas (three) and Virginia (one).

"Every year about a hundred children die from the flu," ABC News chief health and medical editor Dr. Richard Besser said Tuesday on "Good Morning America." 

The flu is already approaching the numbers hit during its peak last year, according to the CDC's latest situation update. 

Over the last couple of years there has been an apparent trend of the flu season's peak approaching earlier than the last, said Besser, who called the trend "worrisome."

"It seems to be peaking at the end of December and it used to be it did not peak until February or March," he said.