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Gastroenteritis (Viral, Bacterial and Parasitic)

Last Updated: Mar 1, 2019


Gastroenteritis is an inflammation of the lining of the stomach and intestines. It is associated with diarrhea and/or vomiting and can be caused by bacteria, parasites, or viruses. Viral gastroenteritis (often called stomach virus or stomach flu) is extremely common, affecting millions of people each year during the cold seasons. Most individuals recover at home without intervention. The complications resulting from gastroenteritis differ based on the pathogen and can include severe dehydration, kidney failure, and coma.

Causes and Risk Factors

Gastroenteritis is caused by a pathogen, usually a virus, and is transmitted when an individual ingests contaminated food or water. Outbreaks typically happen where a Noroviruslarger number of individuals are in close quarters, such as in childcare centers and cruises.

  • Viruses. Viral gastroenteritis is the 2nd highest ailment in the United States. Despite being called “stomach flu,” it is not caused by the influenza virus. Rotaviruses, noroviruses, adenoviruses, sapoviruses, and astroviruses are all associated with viral gastroenteritis and infect adults and children to varying degrees. Norovirus is the culprit behind most viral gastroenteritis epidemics; noroviruses are exceptionally virulent, requiring a median of only 18 viruses to infect. Virus is shed in the stool and vomit of an infected individual and can spread via direct (handshake) or indirect (food) contact.
  • Bacteria. Several types of bacteria including E. coli and salmonella cause gastroenteritis. Food becomes contaminated when it is exposed to fecal matter on the farm, processing plant, or kitchen. Inadequate sanitation, improper food handling, poor refrigeration, and undercooked meats can all lead to bacterial food poisoning.
  • Parasites. Giardiasis is a diarrheal disease cause by the Giardia parasite, the most common intestinal parasite in the United States. Parasitic organisms like Giardia live inside a host organism (animal or human) and are expelled through feces. Infection spreads when hard-shelled Giardia cysts are ingested; as few as 10 cysts may cause illness. Contact with a contaminated surface or untreated water allow the spread of giardiasis.


Diarrhea and vomiting are the most common symptoms of gastroenteritis. Symptoms typically appear within a few days (viral and bacterial) or weeks (parasitic) of becoming infected and may include:

  • Nausea and vomiting
  • Diarrhea
  • Abdominal pain
  • Fever and chills
  • Dehydration (dizziness, thirst, and dark urine)
  • Bloody stools (a symptom of bacterial gastroenteritis)
  • Greasy stools (a symptom of giardiasis)

Diagnosis and Treatment

Gastroenteritis is typically diagnosed based on symptoms alone. A stool sample may be used to identify the causative pathogen. Most patients with gastroenteritis treat their symptoms at home and fully recover within days to weeks. Hospitalization becomes necessary when complications such as dehydration, excessive weight loss, and high fever arise.

  • Hydration. Fluid loss due to repeated episodes of vomiting and/or diarrhea can lead to dehydration, which is common in infants, young children, and the elderly. Regular hydration with simple broths and non-caffeinated liquids is required to replenish the body’s electrolytes. Oral rehydration solutions (e.g. Pedialyte®) are also available without a prescription. If sufficient hydration is not achievable at home, then a doctor administers fluids intravenously.
  • At-home remedies. During bouts of vomiting and diarrhea, frequent intake of small amounts of liquid and simple foods (e.g. rice, potatoes and bread) is recommended. Avoiding foods high in fat and sugar, dairy, caffeine, and alcohol helps to relieve the symptoms of gastroenteritis. Over the counter drugs that slow or stop diarrhea are also available.
  • Prescription medications. Antimicrobial drugs to treat bacterial and parasitic gastroenteritis are prescribed when symptoms become severe, or when complications develop.


The pathogens that cause gastroenteritis are very infectious and can survive outside the host for up to weeks. Extensive handwashing and avoiding contact with sick individuals are critical steps in preventing infection. Contaminated surfaces must be cleaned promptly with a dilute bleach or other disinfecting solution. Proper handling of food; adequate refrigeration; and thorough cooking of eggs, meats, and fish help prevent the growth and spread of harmful bacteria. Extra caution is necessary when traveling to countries where parasites like Giardia are common. Drinking water from untreated lakes and rivers is not advisable without first using an effective water purifier.


  • “Viral Gastroenteritis”. National Digestive Diseases Information Clearinghouse (NDDIC). NIH. Apr 2012. Retrieved Apr 9, 2014.
  • “Viral Gastroenteritis: Heartburn and more”. National Center for Immunization and Respiratory Diseases. CDC. Feb 2011. Retrieved Apr 9, 2014.
  • “Bacterial gastroenteritis”. MedlinePlus. NIH. Jan 2011. Retrieved Apr 9, 2014.
  • “Giardia infection”. MedlinePlus. NIH. May 2012. Retrieved Apr 9, 2014.
  • “Giardia infection”. Centers for Disease Control and Prevention. Jul 2013. Retrieved Apr 9, 2014.
  • Teunis PFM, Moe CL, Liu P, et al. Norwalk virus: how infectious is it?. J Med Virol. 2009. 80:1468-76.
  • “Drinking Water/Camping, Hiking, Travel”. Centers for Disease Control and Prevention. Jan 2012. Retrieved Apr 10, 2014.

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Tina Shahian, PhD

Tina is a writer for Innerbody Research, where she has written a large body of informative guides about health conditions.


A communication specialist in life science and biotech subjects, Tina’s successful career is rooted in her ability to convey complex scientific topics to diverse audiences. Tina earned her PhD in Biochemistry from the University of California, San Francisco and her BS degree in Cell Biology from U.C. Davis. Tina Shahian’s Linkedin profile.


In her spare time, Tina enjoys drawing science-related cartoons.