Skip directly to search Skip directly to A to Z list Skip directly to navigation Skip directly to page options Skip directly to site content

Clinical Overview

Noroviruses were previously called Norwalk-like viruses. They are a group of non-enveloped, single-stranded RNA viruses that cause acute gastroenteritis. Noroviruses belong to the family Caliciviridae that includes sapoviruses, which also causes acute gastroenteritis.

Currently, there are six recognized norovirus genogroups. Three of the genogroups (GI, GII, and GIV) affect humans. More than 25 different genotypes have been identified within these three genogroups. Since 2002, variants of the GII.4 genotype have been the most common cause of norovirus outbreaks.

Noroviruses are named after the original Norwalk strain, which caused an outbreak of gastroenteritis in a school in Norwalk, Ohio in 1968.


A person usually develops symptoms of gastroenteritis 12 to 48 hours after being exposed to norovirus.

Typical symptoms—

  • acute-onset of vomiting
  • watery, non-bloody diarrhea with abdominal cramps
  • nausea

Some people may have low-grade fever, headaches, and myalgias (body aches).

Dehydration is the most common complication, especially in young children and the older adults, that may require medical care.

Symptoms of gastroenteritis usually last 24 to 72 hours. People usually recover completely without any serious long-term problems. But, norovirus illness can be serious, especially for young children, older adults, and people with compromised immune systems. This can lead to severe dehydration, hospitalization, and death.

Not everyone who is exposed to norovirus will get infected. Some people who get norovirus infection may not have symptoms, but they may still shed the virus in their stool.

 Top of Page


Since there are many different types of noroviruses, people can get infected many times during their lifetime. It is possible to develop immunity to specific types. But, it is not known how long immunity lasts. This may explain why so many people of all ages get infected during norovirus outbreaks. Also, whether someone is susceptible to norovirus infection or not may be determined in part by his or her genes.

 Top of Page


Noroviruses are highly contagious. A person with norovirus infection can shed billions of norovirus particles. But, it only takes as few as 18 viral particles to infect another person.

Primarily, noroviruses are spread through—

  • close personal contact with an infected person, or
  • fecal-oral route when a person consumes contaminated food or water.

The virus can also spread through touching contaminated surfaces, objects, or substances.

It is possible for norovirus to spread through aerosolized vomit that lands on surfaces or enters a person’s mouth then he or she swallows it. There is no evidence showing that people can get infected by breathing in the virus.

During outbreaks, norovirus can spread in several different ways. For example, a person who is infected by eating contaminated food in a restaurant can spread the virus to household members through direct contact or by touching and contaminating objects and surfaces.

It is possible for an infected person to shed norovirus before they have symptoms. However, people usually begin shedding the virus once they have symptoms. This may continue for 2 weeks or more after they recover. But, it is not known whether they are still contagious.

More norovirus information for health care providers

 Top of Page

Resources and Related Pages