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There’s a potent flu virus infecting Americans this influenza season -- even healthy people including a marathon runner and Terms of servicebodybuilder have become seriously ill. But although the flu shot isn’t so effective this year, the vaccine will still probably spare you from the most severe symptoms, hospitalization, or at worst, death.

Like most flu seasons, there are a few strains circulating around the country right now, but one of these -- dubbed H3N2 – is notably vicious. At worst, it’s taken the lives of children and healthy adults. The Centers for Disease Control and Prevention (CDC) notes there is "widespread" flu activity in nearly every region it monitors around the country, and H3N2 was the most frequently identified strain reported as of mid-January.

Generally, severe fevers, chills, and fatigue are compelling an unusually high number Americans to seek medical treatment.

“Our hospitals are brimming in the ER,” said Joan Faro, Chief Medical Officer at John T. Mather Memorial Hospital on Long Island, New York, in an interview. “Occupancy rates are through the roof.”

Even medical professionals are taking extra precautions against this season’s virus. Faro said inoculated staff are wearing masks around sick patients -- and that’s something she hasn’t seen before.

“There’s an awareness that there’s something going on, something that is a little bit different than previous years,” Faro said.

The H3N2 virus, though, has hit the U.S. numerous times before. And when it does, “it tends to be a rougher season,” said Susan Donelan, medical director and assistant professor of infectious disease at Stony Brook University’s School of Medicine, in an interview.

“It’s not pretty.”

Already this flu season, 37 children have died in the U.S. from the virus, according to the CDC.

This virus is exceptionally nasty because it tends to change more than other flu viruses during the course of a season. Donelan calls these slight changes, known as “genetic drift,” little tweaks that occur in the viruses’ genes during or between the flu season.

The H3N2 virus' ability to change with time renders the flu vaccine, which is basically a weakened form of several dominant flu viruses, an imperfect match against this year's dominant illness. In essence, those who received the flu shot have spent time preparing to fight a specific invader that, when it finally arrives, ends up presenting itself differently.

The flu vaccine becomes “a near match, but it’s not a perfect match,” said Shane Speights, a dean and associate professor of medicine at New York Institute of Technology College of Osteopathic Medicine at Arkansas State University.

Our flu vaccines are bred in laboratories months in advance, so the virus has ample time to morph during that period. When this happens, the virus can then successfully attack and reproduce in bodies that have been inoculated.

But getting the shot will mitigate the altered viruses’ aggressiveness.

“The vaccine certainly still provides a lot of benefits,” explained Speights. “It’s still enough for your body to mount a response.”

“It starts creating infantry cells so that when you come in contact with the real thing, it has some resistance to fight it off,” said Speights.

And this bit of resistance, said Donelan, “can still keep people from getting really ill, and if hospitalized, can keep them from dying.”

For that reason, even if it’s late January or early February -- which is quite late in the flu season – Speights emphasized that “It’s not too late to get the vaccine. At minimum, this will “give your body a look at [the virus],” he said.

And that seems like wise advice for a strain that can morph quickly, partially outwitting our carefully-developed vaccines.

“Influenza is a pretty clever organism,” said Donelan.