Causes and Risk Factors of Stomach Flu (Viral Gastroenteritis)

Viruses can spread quickly and may cause diarrhea and vomiting

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Viral gastroenteritis, commonly called the stomach flu, can be caused by several different types of viruses. It is also sometimes caused by bacteria or parasites. In many cases, viral diseases are highly contagious and are spread through the fecal-oral route. This is when the virus is shed in the stool of an infected person and then gets on hands or other surfaces. When an uninfected person touches those surfaces and then touches their nose, eyes, or mouth, they can then become infected also. Good handwashing practices and other precautions may help stop the spread of viruses.

how long stomach flu symptoms last
Illustration by Cindy Chung, Verywell

Common Causes

Viral gastroenteritis is caused by one of several different types of viruses. Gastroenteritis can also be caused by bacteria (which is usually called food poisoning) or parasites (parasitic infection).

It’s not usually known which type of virus is causing the symptoms without testing and testing isn’t usually done unless there are severe symptoms.

Because having loose stools and vomiting from infectious diarrhea can come on suddenly and usually only lasts for a few days before going away on its own, most people don’t go to a doctor or get treated for gastroenteritis. For that reason, it’s usually not known what type of virus or bacteria caused the illness, although sometimes it’s known where the infection came from because another person was sick first.

There are several different viruses that are most often the cause of viral gastroenteritis.

Norovirus

Norovirus (sometimes known as the Norwalk virus) is the virus most often responsible for the big outbreaks of diarrhea and vomiting that get passed around places where people are in close contact like schools and, infamously, cruise ships. Noroviruses are spread through food and drink and are highly contagious. People can become ill after eating or drinking something that is contaminated by a norovirus. Even though they are spread in the food supply, the noroviruses are not considered a type of food poisoning. 

Gastroenteritis that's caused by the norovirus is usually an illness that is more common during the winter months. Norovirus causes vomiting, diarrhea, and stomach cramps that can last anywhere between one and three days. It can take several more days after diarrhea and vomiting stop to feel better.

Once a person is infected, they can shed the norovirus and spread it to others through physical contact like shaking hands or on utensils or in food. A person who is infected with norovirus can spread it before they become sick with symptoms, as well as for up to 8 weeks after, which makes outbreaks spread quickly and difficult to contain.

Rotavirus

Worldwide, rotavirus is the most common cause of diarrhea in infants and children. In developing countries, it is a serious concern because it can lead to rapid dehydration in babies and young children. People who have suppressed immune systems because of illness or medications are also at higher risk of becoming infected. Symptoms of rotavirus tend to be less severe in adults.

There is a vaccine for rotavirus, which has had the effect of lowering the number of cases of rotavirus that occur in the United States.

Vaccination does not provide full immunity from rotavirus. However, after being vaccinated children are less likely to become sick from rotavirus and if they do become ill, the symptoms are usually less severe. Becoming exposed to rotavirus and sick from it may not protect against getting the virus again in the future.

Rotavirus causes symptoms about two days after being exposed to it. Rotavirus is shed from an infected person through their stool. Young children tend to touch their diaper area and then their faces or other areas and that spreads the virus.

People who are infected with rotavirus can spread it before they start to have symptoms. Rotavirus can be spread at any time of year but is more common in the winter and the spring.

Astrovirus

Infection with an astrovirus can start to cause signs and symptoms between three and five days after being exposed. The symptoms can include diarrhea, headache, and abdominal pain. Illness caused by an astrovirus is usually milder in nature than the symptoms caused by infection with norovirus or rotavirus.

Infections with an astrovirus tend to happen more often in the winter months and are more common in children and infants than in adults. People who have compromised immune systems or the elderly whose immunity to the virus has decreased over time are also at increased risk of infection.

After being infected with an astrovirus, most people tend to feel sick for anywhere between one and four days. Even after feeling better, an infected person can still shed the virus in their stool and pass it on to others.

Adenovirus

Adenoviruses can cause many different types of illnesses, including viral gastroenteritis. Infection with adenovirus is more common in children under the age of 2 years but can also occur in older children and adults.

The symptoms of an infection with an adenovirus are usually milder than they are with other viruses. Signs and symptoms can start anywhere between three and 10 days after becoming infected. Adenoviruses can make a person feel ill for a little longer than other viruses, usually between 1 and 2 weeks. 

Infections with adenovirus can occur at any time of year but tend to be slightly more common in the summer. These viruses spread from person to person, usually through the stool from an infected person that’s on hands or on other surfaces. The virus can continue to be shed in the stool for several weeks after the illness is over. Children and people who have compromised immune systems because of medication or illness are more likely to become sick with an adenovirus.

Lifestyle Risk Factors

Viral gastroenteritis tends to be more common in babies and children. It spreads by the fecal-oral route.

The virus leaves the body through the stool of an infected person (who may or may not currently have symptoms). After going to the bathroom or changing a diaper, the virus may be on a person's hands. If hands are not washed thoroughly the virus can pass from hands onto foods or other surfaces. An uninfected person might touch a surface or eat food that has the virus on it, or touch their nose or face, and become infected.

One of the keys to stopping the spread of infections is the consistent use of good hygiene. Alcohol-based hand sanitizers may be helpful in certain situations when soap and water are not available but they are not as effective at eliminating viruses as good hand washing. It's important to wash hands well even when no one appears ill because a virus can be spreading even before symptoms start.

Keys to Preventing Spread of Viral Gastroenteritis

  • Washing hands for 20 seconds with soap and water, especially: after changing a diaper, after going to the bathroom, before preparing food, and before eating.
  • Avoiding touching the face with unwashed hands.
  • Don't share food or utensils with other people.
  • Dispose of dirty diapers or other items with stool on them with care.
  • Avoid close contact with people who are already ill.
  • After an illness, clean surfaces that may have the virus on them with detergent and chlorine bleach.
  • Don't prepare any food for other people until at least 2 or 3 days after diarrhea/vomiting stops.
  • Don't eat food that is prepared by someone who is ill with diarrhea/vomiting.
  • Stay home and away from other people when ill.

The fecal-oral route tends to make most people cringe because it implies close contact between stool or feces and the face. Not washing hands well is usually how the viruses that cause illnesses are spread.

However, it’s important to remember that the viruses that cause illness are spread quite easily, especially in daycare centers, schools, or adult care facilities. Those who care for children and adults who wear diapers, especially, need to take care to wash hands carefully with soap and water. Babies and children should have their hands washed often, especially if they are touching their diaper area.

Many of the viruses that cause viral gastroenteritis can be shed before a person becomes sick and then for a long time after recovery. This is why it’s important to stick with hand washing and good disinfecting methods to keep bathrooms and diaper changing areas clean even when no one is currently ill or having diarrhea.

In the case of rotavirus, it is important for children to receive the two vaccinations for this virus. It is the childhood vaccinations that have dramatically lowered the number of cases of gastroenteritis from rotavirus since 2006.

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Additional Reading

  • Boyce TG. Overview of gastroenteritis. Merck Manual: Professional Version website. May 2017.  

  • Esona MD, Gautam R. Rotavirus. Clin Lab Med. 2015 Jun;35(2):363-91. DOI: 10.1016/j.cll.2015.02.012.

  • National Center for Immunization and Respiratory Diseases, Division of Viral Diseases. Rotavirus. Centers for Disease Control and Prevention. 23 April 2018.

  • Robilotti E, Deresinski S, Pinsky BA. Norovirus. Clin Microbiol Rev. 2015 Jan;28(1):134-64. DOI: 10.1128/CMR.00075-14.

  • The National Institute of Diabetes and Digestive and Kidney Diseases Health Information Center. Viral Gastroenteritis (“Stomach Flu”). National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). May 2018.