GREEN BAY, Wis. (WBAY) It's likely already struck you or someone you know.
Nebraska is experiencing a high activity level in flu-like illness so far this season.
It's safe to say a lot of people have been sick with the flu this season.
The Centers for Disease Control and Prevention lists much of the U.S. in the high flu activity category, including Wisconsin.
But what's different this year, and what's still puzzling to doctors, is the kind of influenza more people are getting.
The strain of flu that's been most active so far is the one most likely to affect more kids.
When you've got a high fever, aches and a virus working hard on your lungs, you just feel awful, no matter what strain of influenza you have.
But to doctors -- and to parents -- that strain does matter a little more.
"The main difference is the populations that are susceptible. Usually kids are more susceptible to B strain, and adults are more susceptible to A strain," says Prevea Health infectious disease physician Dr. Edward Morales.
Of the two common strains of the flu, almost every year strain A strikes first.
Strain B usually comes later in the season, often in February.
But this year, Prevea Health centers statewide report 30 times more cases of that B strain than this time last year, with 151 cases compared to just five a year ago.
Dr. Morales says the strains are both incredibly contagious, but kids, especially young children, are most susceptible to the strain surging right now.
He says that may be partly because they have less-developed respiratory tracts or immune systems which aren't that strong yet.
"There may be a situation with population, like children might be a little less inclined to use hand sanitizers or hand washing and good infection control practices, where adults may be more so, so that might be the reason why it seems to be more in one population than the others as well," explains Dr. Morales.
But exactly why the strains are flip-flopped the medical community can't really answer yet.
Doctors do say we're not likely near a peak yet so getting a flu shot is still recommended and can still give you protection.
Dr. Morales says the vaccine this year does cover both A and B strains.
"For most vaccines, complete immunity usually (takes) two weeks or so, maybe as much as three weeks, but you start getting immunity as soon as you get the vaccine, so you start actually developing that protection right away, so it's worthwhile," says Dr. Morales.
He says adults and children can fall sick with either strain.
When asked whether he expects to see more A strain influenza in the coming weeks, Dr. Morales says, "I think the flu season is really unpredictable, and people don't always know what's going to happen."
He says fevers for the virus can last for several days, and kids shouldn't be going to school or daycare until fevers are completely gone. Most schools ask parents not to let children return to school until they're fever-free for at least 24 hours.