Why You Have the Stomach Flu After Getting a Shot

It arrives in the middle of the night — that feeling ... oh, that horrible feeling. You jump out of bed, hoping you'll make it to the bathroom in time. Oh, no (oh, yes) — it's the stomach flu, a.k.a. the pesky "stomach bug."

You probably even know who you gave you the stomach bug. And as contagious as it is, you probably passed it on to someone else before you even realized you had it.

But there's one problem: You got a flu shot this year. In fact, you get a flu shot every year. So how in the world was it even possible for you to get the stomach flu?

Well, there's a simple reason: The stomach flu isn't really the flu. You don't actually have the flu — you have a stomach bug.

According to the Centers for Disease Control and Prevention, what we call the "stomach flu" is not influenza (the real flu) at all.

To avoid this confusion, the stomach flu really should be called by its actual name — gastroenteritis. But most people are likely to continue to call it "the flu" or "the stomach flu."

When you have gastroenteritis, your stomach and intestines become inflamed. This inflammation leads to nausea, vomiting, and watery diarrhea. You may also have stomach cramps, a headache, and a fever.

Gastroenteritis can be caused by bacteria or a virus, including such horrible sounding bugs as rotaviruses, noroviruses, adenoviruses or astroviruses. When you hear of outbreaks of norovirus on a cruise ship (or other places where lots of people gather in close quarters), that's gastroenteritis.

True influenza, including the seasonal flu we get flu shots for, is also caused by viruses — just different ones. The seasonal flu viruses actually change from year to year, which is why we have to get a new flu vaccination each year.

Other forms of influenza, such as swine flu or avian flu, are caused by specific viruses. Like their seasonal flu cousins, those viruses are not the same as those which cause gastroenteritis.

When you're worshipping that porcelain throne and your body is wracked with chills, pain, and gastrointestinal upset, it probably doesn't make one bit of difference to you whether stomach flu is misnamed.

But if you use this stomach ailment as an excuse not to get a flu shot (because you got a shot one year and ended up with gastroenteritis anyway), then this distinction is very important. Remember — your flu shot protected you from influenza. It didn't have anything to do with that case of gastroenteritis.

And the next time you hear about someone who got the "stomach flu," pass the word. The stomach flu is not really the flu. It's gastroenteritis.