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Upset Stomach: Is it Food Poisoning or Stomach Flu?

25 July 2022

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People who have found themselves suffering from intense stomach pain or on the toilet dealing with diarrhea will often scour the internet to see if they’re suffering from a stomach bug or food poisoning. Considering the two gastrointestinal illnesses share similar symptoms, it can be hard to tell the difference. Knowing which infection you have can help provide guidance on how long your symptoms will last and how to go about managing your illness.

What is the difference between stomach flu and food poisoning?

At a high level, the stomach flu and food poisoning fall under the broad category of gastroenteritis, a medical term to describe any inflammation (irritation) of your intestines.

The source of infection is the main difference between the stomach flu and food poisoning. In most cases, the stomach flu is caused by a viral infection, while food poisoning is caused by bacteria or a parasite.

The other main difference is symptom onset – food poisoning can develop in a matter of hours after eating contaminated food. The stomach flu, on the other hand, can take several days to cause symptoms after becoming infected.

Stomach flu

The terms stomach flu and stomach bug are both interchangeably used to describe viral gastroenteritis, which is a viral infection that causes inflammation and swelling of the lower gastrointestinal tract – stomach, small intestine and large intestine. Some people can become ill from bacteria, parasites or toxins, although viruses are the most common cause of the stomach flu.

It should be noted the stomach flu is unrelated to the influenza that causes upper respiratory systems. 

Here are the most common viruses that cause the stomach flu:

  • Norovirus: The most common cause of viral gastroenteritis in adults, symptoms may not begin until one to two days after becoming infected. The norovirus can last up to three days. 
  • Rotavirus: More common in children than adults, rotavirus symptoms begin two days after becoming infected. Symptoms may clear up in a few days but may also last up to a week. Children can receive a rotavirus vaccine to prevent infection.
  • Adenovirus: The incubation period of adenovirus is longer than the other three viruses listed, as it can take up to 10 days to become symptomatic. From there, symptoms can last one to two weeks.
  • Astrovirus: This type of virus usually produces symptoms four to five days after becoming infected and lasts a few days.

Diarrhea is the main symptom of the stomach flu, and people also experience nausea, vomiting, stomach cramps, fever, headaches and dizzy spells. The virus doesn’t allow the GI tract to absorb water from the foods you eat, thus bowel movements become increasingly more liquid than normal.

A combination of diarrhea and vomiting leads to dehydration in some people with the stomach flu. The loss of body fluids and electrolytes leads to dehydration and can cause dry mouth, lightheadedness, decreased urine output, dark-colored urine or fatigue.

Food poisoning

Food poisoning occurs when you consume food or water that is contaminated with viruses, bacteria or parasites. There are more than 250 types of food poisoning, and it affects about 48 million people a year, according to the National Institute of Diabetes and Digestive and Kidney Diseases.

Symptoms include diarrhea, stomach cramps, fatigue, fever, chills, muscle aches and excessive sweating. Like the stomach flu, dehydration is a common complication of food poisoning. In some cases, a bacterial infection from E. coli can cause hemolytic uremic syndrome, a condition in which destroyed red blood cells damage the kidney. 

Viruses that cause food poisoning:

  • Norovirus
  • Hepatitis A

Bacteria that cause food poisoning:

  • Campylobacter 
  • Clostridium (C. perfringens and C. botulinum)
  • Escherichia coli (E. coli)
  • Listeria monocytogenes (listeria)
  • Salmonella NIH external link
  • Staphylococcus aureus (staph)
  • Vibrio (parahaemolyticus and vulnificus)

Parasites that cause food poisoning:

  • Cryptosporidium 
  • Giardia
  • Toxoplasma gondii

Several of the most common causes of food poisoning have a short incubation period, including staph (30 minutes to 8 hours), salmonella (6 to 48 hours) and clostridium perfringens (8 to 16 hours).

Is food poisoning contagious?

Food poisoning isn’t usually contagious since it spreads through food. However, there are certain cases where food poisoning may spread from person-to-person. This includes the spreading of microorganisms to food or drinks by infected people who don’t wash their hands after using the bathroom.

In most instances, though, harmful microorganisms are already present on foods such as fruits, vegetables and raw meat.

How is the stomach flu spread?

Viral gastroenteritis, on the other hand, is highly contagious. The infection spreads by close contact, such as sharing items with an infected person or touching your mouth after coming in contact with a contaminated surface (doorknobs, handles, countertops, etc.).

Norovirus is the most common way the stomach flu spreads. In most cases, the virus is spread in restaurants or food distribution centers where workers handle ready-to-eat food improperly. Buffets, such as those found at resorts or cruise ships, tend to be most affected as workers can touch food with their bare hands and unknowingly spread the virus.

What to eat when you have food poisoning or the stomach flu?

Whether you have come down with a stomach bug or have food poisoning, here are a few tips to follow. 

Losing your appetite is one of the first things that goes with viral gastroenteritis or food poisoning. But, with the loss of fluids and electrolytes, eating and drinking is still crucial when fighting against GI illnesses.

For starters, you can follow the BRAT diet, which stands for bananas, rice, applesauce and toast. These foods won’t help your symptoms, rather, they are gentle enough to where they won’t upset your stomach any further.

Other foods that may be kind to your stomach include cereal (no milk), saltine crackers, oatmeal or broth. Saltine crackers are beneficial to provide both calories and salt lost from dehydration. However, you shouldn’t eat these foods for more than a day or two, as they lack the nutrients your body needs for energy and for other daily routine tasks.

In terms of liquids, drink plenty of water to help replenish any fluids lost. Sports drinks are a good source of electrolytes, but many versions are loaded with added sugars.

While gentle foods can help provide energy while your body fights the infection, certain foods may actually make your symptoms worse. For example, caffeine is a stimulant and can lead to more frequent bowel movements, while foods high in fat can disrupt your GI tract. Even milk can be hard to digest when you’re sick due to the presence of lactose and can compound your issues.


While an upset stomach is no fun to deal with in the short term, most cases are mild and will resolve in 24 to 48 hours by treating it with rest and plenty of fluids. Some cases are more severe, though, and may lead to severe dehydration. Monitor your symptoms, and if you feel fatigued, overly thirsty and experience diminished urine output, contact your doctor or go to your nearest urgent care.


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