Diarrhea and other gastrointestinal (GI) problems are sometimes symptoms of influenza (flu), although not every person with the flu experiences them.

While the flu is a common cause of respiratory illness, it only sometimes causes diarrhea. Researchers do not know why or how this may occur.

According to the Centers for Disease Control and Prevention (CDC), diarrhea during the flu is more common in children than in adults.

This article looks at why influenza may cause diarrhea, stomach flu, and vomiting. It also looks at other flu symptoms and treatments.

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The flu virus typically infects people via a respiratory route. However, GI symptoms, including diarrhea, vomiting, and abdominal pain, sometimes occur. GI symptoms can indicate severe flu and represent a secondary infection.

It is common for the flu virus to spread to the GI tract after a respiratory infection. However, researchers do not know how it is spread.

Researchers suggest that a person with the flu may experience diarrhea as a result of taking flu medications, which can irritate the gut, or because of a direct viral effect or secondary bacterial infection.

Researchers that identified flu viruses in feces estimate that these reach and affect the GI tract due to:

  • the flu virus replicating in the intestinal cells
  • a person swallowing the flu virus
  • affected intestinal immune cells

Stomach flu, also known as viral gastroenteritis, is a highly contagious intestinal infection. Symptoms include diarrhea, vomiting, and stomach cramps.

Stomach flu is not the same as influenza, which primarily affects the respiratory system. Stomach flu mainly affects the intestines and stomach.

A person can develop stomach flu from different viruses, including:

  • rotavirus, which is most common between April and December
  • norovirus, which is the most contagious form of stomach flu
  • astrovirus, which typically affects people with weakened immune systems
  • adenovirus, which can cause a wide range of symptoms

A person will usually develop symptoms following contact with someone who has the stomach flu. They may also develop symptoms from food and water containing the virus.

Other symptoms of the flu may include:

  • fever
  • chills
  • headaches
  • stuffy or runny nose
  • cough
  • muscle aches
  • sore throat
  • fatigue

Individuals most commonly treat the flu at home with over-the-counter (OTC) medications, such as painkillers, decongestants, and cough medications.

Resting, ensuring enough fluids, and managing body temperature can also help reduce the severity of symptoms.

In some instances, a doctor may prescribe antiviral drugs that can reduce the severity of flu symptoms. They can also reduce the overall amount of time a person is sick by 1 or 2 days. The drugs work most effectively if a person takes them within 2 days of getting the infection.

People who are at high risk of serious complications from flu viruses might especially benefit from antiviral medication. People at higher risk of complications include:

A person can usually treat diarrhea using OTC medications, such as bismuth subsalicylate (Kaopectate or Pepto-Bismol) and loperamide (Imodium).

However, if a person has a fever or notices blood in their stools, it may indicate a different type of infection, such as bacteria or parasites. For instances such as these, a doctor may not recommend OTC medications.

It is important to treat dehydration, which can result from diarrhea. A person should drink plenty of water to replace lost fluids and consider other options to replace electrolytes, including:

  • sports drinks
  • caffeine-free soft drinks
  • broths
  • fruit juices

If diarrhea lasts longer than 2 days, a person should consider contacting a doctor.

There are several ways a person can actively prevent the flu. These include:

  • Flu vaccines: Flu vaccines protect against the most common flu viruses. Anyone 6 months or older should receive a flu vaccine, as it can also help prevent high risk people from serious flu complications and illness.
  • Stopping the spread: Individuals should avoid contact with people who are sick with the flu, if possible. Similarly, if a person has the flu, they should limit their contact with others. Covering the nose and mouth while coughing or sneezing and not touching the face can also limit the spread of the flu.

People should also wash their hands thoroughly and regularly as well as disinfect surfaces.

Some people with the flu may experience diarrhea as a symptom. The flu virus may spread to the GI tract as a secondary virus and cause diarrhea. Researchers are unsure of exactly how and why this occurs.

Stomach flu, or viral gastroenteritis, differs from the flu. Stomach flu primarily affects the intestines and stomach, while the flu primarily affects the respiratory system. The two viruses have different causes.

A person can usually treat the flu and diarrhea at home using OTC medications. However, if complications occur and symptoms become severe, they should seek medical attention.

A person may prevent the flu by avoiding contact with the virus and by receiving a flu vaccine.