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(Acute Nonbacterial Gastroenteritis; Caliciviruses; Food Infection; Norwalk Virus; Norwalk-like Virus; Small Round Structure Viruses [SMRVs]; Stomach Flu; Viral Gastroenteritis)

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Noroviruses refer to a group of viruses that causes inflammation of the stomach and intestines, called gastroenteritis (or, more commonly, the “stomach flu”). In the US, noroviruses are the second leading cause of illness (the common cold is the first). Outbreaks have occurred in settings such as cruise ships, restaurants, nursing homes, hospitals, and other locations where the virus can spread quickly to a large group of people.

The Digestive Tract


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The highly contagious noroviruses are spread by fecal-oral contamination of water and food.

Infection can occur as a result of contaminated:

  • Municipal water supplies, recreational lakes, swimming pools, wells, water stored on cruise ships, among other sources
  • Raw (or improperly steamed) shellfish, especially clams and oysters
  • Food and drinks (due to infected food handlers who either do not wash their hands or wash their hands improperly after using the bathroom)
  • Surfaces (eg, touching a door knob and then placing hands in mouth)

Norovirus can also spread by direct contact with an ill person, such as in a daycare center or nursing home.

Risk Factors

A risk factor is something that increases your chance of getting a disease or condition.

The following factors increase your chance of developing noroviruses. If you have any of these risk factors, tell your doctor:

  • Age: more common in adults and children than the very young
  • Being exposed to a contaminated water supply (eg, recreational lake)
  • Consuming contaminated foods or liquids
  • Touching contaminated surfaces
  • Taking care of someone who is infected with the virus (eg, in a nursing home or daycare center)
    • Note: A person is contagious from the start of symptoms to at least three days after recovery (and sometimes up to three weeks).

Even if you have been infected with norovirus in the past, you can become ill again if it is a different strain or if it is over 24 months since your last exposure.


If you experience any of these symptoms do not assume it is due to norovirus. These symptoms may be caused by other, less serious health conditions. If you experience any one of them, see your physician.

  • Nausea
  • Vomiting
    • An infected person may vomit often (sometimes violently and without warning) during one day.
  • Diarrhea
  • Abdominal pain
  • Headache
  • Low-grade fever
  • Chills
  • Muscle aches
  • Tiredness
  • Dehydration
    • This may require medical attention, especially in children, the elderly, and those with compromised immune systems. You can prevent dehydration by drinking a lot of fluids, including water and juice.

After exposure to the virus, symptoms typically appear within 24-48 hours, but you can begin feeling ill as early as 12 hours. Symptoms usually last 24-60 hours.


Your doctor will ask about your symptoms and medical history, and perform a physical exam.

Diagnosis can be made based on a stool specimen; typically, though, your doctor can determine this illness without ordering any laboratory tests.


Currently, there are no treatments for gastroenteritis caused by noroviruses. Because gastroenteritis is caused by a virus, antibiotics cannot cure it, and there are no antiviral medications or vaccines. Typically, though, the illness is brief. The only complication would be dehydration due to vomiting and diarrhea. In certain groups of people (eg, the young and the elderly), this may require hospitalization to replenish fluids.


Noroviruses can survive freezing temperatures, as well as temperatures up to 140°F. The virus can also live in water with up to 10 parts per million of chlorine, which is much higher than what public water supplies have. There are ways, though, to limit exposure.

To help reduce your chance of getting noroviruses take the following steps:

  • Wash hands thoroughly after using the bathroom (or changing diapers) and before handling food or eating. If you are caring for someone who is infected, make sure the person (especially a child) thoroughly washes his or her hands.
  • Wash fruits and vegetables; steam oysters and clams.
  • Do not prepare food if you have symptoms, and wait three days after you have recovered before handling food again.
  • Throw away contaminated food.
  • If you are ill or caring for someone who is ill, immediately clean and disinfect contaminated surfaces using bleach cleaner. Remove and wash (using hot water and soap) soiled linens.
  • If you are sick, do not attend work. Staying home will prevent you from passing the virus to others.
  • If you work in a healthcare facility, isolate sick individuals to reduce the virus from spreading.


National Institute of Allergies and Infectious Diseases

US Centers for Disease Control and Prevention

US Food and Drug Administration


BC Health Guide

Communicable Disease Control Unit

Health Canada


Causes and symptoms of norovirus infection. Minnesota Department of Health website. Available at: http://www.health.state.mn.us/divs/idepc/diseases/norovirus/basics.html . Accessed June 11, 2007.

Foodborne diseases: norovirus infection. National Institute of Allergies and Infectious Diseases website. Available at: http://www3.niaid.nih.gov/healthscience/healthtopics/norovirus/diagnosis.htm . Accessed June 12, 2007.

Norovirus: food handlers. National Center for Infectious Diseases: Respiratory and Enteric Viruses Branch. Centers for Disease Control and Prevention website. Available at: http://www.cdc.gov/ncidod/dvrd/revb/gastro/norovirus-foodhandlers.htm . Accessed June 11, 2007.

Norovirus in healthcare facilities fact sheet. National Center for Infectious Diseases: Respiratory and Enteric Viruses Branch. Centers for Disease Control and Prevention website. Available at: http://www.cdc.gov/ncidod/dhqp/id_norovirusFS.html . Accessed June 11, 2007.

Norovirus: Q&A. National Center for Infectious Diseases: Respiratory and Enteric Viruses Branch. Centers for Disease Control and Prevention website. Available at: http://www.cdc.gov/ncidod/dvrd/revb/gastro/norovirus-qa.htm . Accessed June 11, 2007.

Norovirus: technical fact sheet. National Center for Infectious Diseases: Respiratory and Enteric Viruses Branch. Centers for Disease Control and Prevention website. Available at: http://www.cdc.gov/ncidod/dvrd/revb/gastro/norovirus-factsheet.htm . Accessed June 11, 2007.

The Norwalk virus family. In: The Bad Bug Book: Foodborne Pathogenic Microorganisms and Natural Toxins Handbook . US Food and Drug Administration; 1992. Center for Food Safety and Applied Nutrition website. Available at: http://www.cfsan.fda.gov/~mow/chap34.html . Accessed June 11, 2007.

Preventing norovirus. Minnesota Department of Health website. Available at: http://www.health.state.mn.us/divs/idepc/diseases/norovirus/prevention.html . Accessed June 11, 2007.

Last reviewed April 2008 by David L. Horn, MD, FACP

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.

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