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(NEXSTAR) – While respiratory illnesses are getting a lot of attention right now (and for good reason), you may find yourself afflicted with a different issue: a stomach bug.

Gastroenteritis – often called stomach flu – is very common, according to the Centers for Disease Control and Prevention, with millions of cases reported every year. We also routinely hear warnings of listeria, E. coli and salmonella outbreaks, all of which can cause food poisoning.

How can you tell them apart?

The first big difference is that stomach flu is caused by a virus (norovirus, not influenza), while food poisoning is caused by bacteria in food that’s improperly cooked or refrigerated. But that’s not something that will help you tell them apart at the moment when you start to feel sick.

The symptoms are also confusingly similar: diarrhea, nausea, vomiting, chills, fever and stomach pain are associated with both the stomach flu and food poisoning, according to the CDC.

The types of symptoms may not differ much, but their onset does. While viruses that cause stomach flu usually have an incubation period of one to two days, food poisoning is usually felt within a few hours.

“Most people don’t have any problem thinking back to what they’ve just eaten, and realize maybe that egg salad was sitting out for too long,” said Donald Ford, chairman of Family Medicine at Cleveland Clinic.

If multiple people are feeling sick two to six hours after eating together, that’s a sign it’s food poisoning, said Ford. With stomach flu, which can spread person-to-person, you may start to feel ill 12 to 48 hours after seeing someone who was infected.

With stomach flu, most people start to feel better in one to three days, says the CDC.

For food poisoning, the CDC recommends seeing a doctor if you have severe symptoms, including bloody diarrhea, fever above 102 degrees, frequent vomiting, dehydration or symptoms lasting more than three days.

In both cases, the sickness often runs its course on its own with rest and hydration.

Fluids are especially important, said Ford. “When your body is trying to get rid of what’s in there, it’s going to throw out the good with the bad. It’s pushing out a lot of fluid along with the toxins that are making you ill. You have to be super aggressive about replacing them.”