Here's why healthy people really die from the flu

America is in the midst of record-breaking influenza outbreak, one that experts say rivals the 2009 swine flu pandemic. According to the Centers for Disease Control and Prevention, it’s an epidemic that’s active in 48 states and Puerto Rico and has led to 17,000 hospitalizations in the past four months alone.

Beyond its pervasiveness, the infection has proven to be alarmingly lethal, killing (in combination with pneumonia) as many as 4,000 Americans a week. While the highest-risk populations remain infants, the elderly, and those with compromised immune systems, young and healthy individuals can experience a severe flu infection too. If left untreated, this can turn deadly.

(Photo: Getty Images)
(Photo: Getty Images)

Anyone who doubts this need look no further than the internet. In the last three months, stories of young, seemingly fit people falling gravely ill from the flu have begun to populate the news cycle with unsettling regularity.

Most recently, there was Heather Holland, a 38-year-old mother of two in Texas, who ended up in the intensive care unit after deciding to forego Tamiflu, a flu medicine. Two days after arriving in the ER, Holland died from organ failure. Her husband told a local Dallas news station that his wife was otherwise healthy with no underlying medical conditions.

Then in December there was the story of 21-year-old “fitness buff” Kyler Baughman, whose parents say he looked “rundown” after contracting the flu around Christmas. Baughman, who was a bodybuilder studying to become a personal trainer, decided to stick it out. Twenty-four hours after being rushed to the ER, he was dead.

While stories like this are prevalent, they’re not necessarily new. Back in 2015, a 26-year-old newlywed in Wisconsin named Katie McQuestion was rushed to the hospital with a dangerously low temperature and fast heart rate. Despite getting a flu shot, she’d contracted the illness a few weeks prior and watched it get progressively worse. Twelve hours after arriving at the hospital, McQuestion was pronounced dead.

In all three cases, there’s a common denominator— a lethal, often- overlooked complication of influenza (as well as other illnesses), one that kills as many as 500,000 Americans every year: sepsis.

At its simplest, sepsis is an overreaction — a response to an infection that’s too extreme and subsequently makes things worse. More specifically, it’s the result of an excess of “inflammatory mediators” (cells meant to fight infection) entering the bloodstream and sending the body into overdrive.

This abundance of inflammation in the body triggers what scientists call a “cascade of changes” that cause tissue damage, organ failure, and ultimately — if not caught early enough — death. While sepsis — or more specifically septic shock (the most severe stage) — is not the only life-threatening complication of this flu, it’s one that can be stopped in its tracks if it’s caught early.

Philippe Bauer, MD, a pulmonologist and researcher at the Mayo Foundation for Medical Education and Research, says one of the biggest obstacles to successfully treating sepsis is getting people to recognize its severity. “Sepsis is a life-threatening organ dysfunction … it’s an explosive response to the infection,” Bauer tells Yahoo Lifestyle. “It’s an emergency. Within hours you can develop organ failure and die.”

While, in most people, a release of inflammatory cells is balanced out by normal ones, in those with sepsis — for reasons that are still unclear — the body can’t keep up. The inflammatory cells far outnumber the normal ones. Since these cells are designed to quickly ward off an infection, Bauer says, they are extremely potent. The resulting “exuberant” reaction can cause death within hours.

So what does someone look for if they want to prevent this?

“From a clinical standpoint, the most obvious feature is that you don’t look right,” says Bauer. “The brain is involved very early, so you usually have some mental status change — you look confused, drowsy, dizzy. You may be unable to walk.” Next there is difficulty breathing, increased heart rate, and decreased temperature.

It’s this last one that seems to dissuade people from getting treatment. “The problem is when you don’t have a fever, people say it can’t be that bad,” Bauer says. “They think, ‘Maybe it’s nothing.’” But Bauer says it’s just the opposite. Although sepsis can cause a slight fever, he says the patients he sees almost always have the opposite: a low body temperature.

Aside from ignoring a lack of fever and watching out for mental shifts like dizziness and confusion, Bauer says people who suspect a loved one may have sepsis shouldn’t think twice. If caught early enough, sepsis can be treated through IV fluids and medicine. But once organ failure has begun, it’s generally too late. “Sepsis is acute,” says Bauer. “Minutes count.”

Editor’s Note: An earlier version of this article stated that Katie McQuestion died this year. McQuestion actually passed away in 2015.

Read more from Yahoo Lifestyle:

Follow us on Instagram, Facebook, and Twitter for nonstop inspiration delivered fresh to your feed, every day.