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Norovirus (Norwalk-like virus, Small Round Structured Virus)

Epidemiology of Infectious Diseases: Norovirus

Causal agent
Noroviruses (genus Norovirus, family Caliciviridae) are a group of related, single stranded RNS, non-enveloped viruses that cause acute gastroenteritis in humans1.

Previously referred to as Norwalk-like viruses (NLVs), caliciviruses and as small round structured viruses (SRSVs).

Common clinical features
Norovirus infection usually presents with the acute onset of vomiting, watery non-bloody diarrhoea, abdominal pain, headache, nausea and occasionally low-grade fever.

Symptoms typically last between 12 to 60 hours.

Complications include dehydration, usually affecting the young and elderly and may require hospital treatment.


  • Endemic worldwide.
  • Noroviruses are the most common cause of outbreaks of gastro-enteritis in hospitals and also cause outbreaks in other settings including schools, hotels, nursing homes and cruise ships4.
  • The Health Protection Agency (HPA) received reports of between 130 and 250 norovirus outbreaks each year.
  • In 2004 there were 3,133 laboratory confirmed cases of norovirus infection in the England and Wales.
  • However, the true incidence of infection is likely to be far higher. The HPA estimate that Norovirus affects between 600,000 and 1 million people in the UK each year. 

Gastrointestinal tract of humans.

Mode of transmission

  • Person to person via the faecal-oral route (e.g. infected food handlers).
  • Ingestion of contaminated food and water . The most commonly contaminated food is shellfish that have concentrated the virus from sewage-contaminated waters3.
  • Environmental contamination (contaminated surfaces, e.g. toilets).  Noroviruses may remain viable for many days on carpets or curtains3.
  • Evidence exists for transmission due to aerosolisation of vomitus that results in droplets contaminating surfaces or entering the oral mucosa and being swallowed, and has been suggested to explain the rapid spread in hospital settings1,2.

Incubation period
12-48 hours  (median in outbreaks is 33-36 hours)1.

Period of Communicability
Until 48 hours after the resolution of symptoms.

Noroviruses are highly infectious and the infecting dose is very low.

Prevention and control
Follow correct food hygiene practices for food preparation and cooking in domestic and commercial kitchens.

Good standard of infection control in hospitals and nursing homes.

Exclude cases in groups with risk of further transmission until 48 hours after resolution of diarrhoea and vomiting3.

These include, food handlers, staff at healthcare facilities who have direct contact, or contact through serving food, with susceptible patients or patients at risk of serious illness.
Children <5 years who attend child day care facilities.
Children or adults (who may find it difficult to implement good standards of personal hygiene e.g. Those with learning disabilities or special needs.


  1. Centers for Disease Control and Prevention, CDC Technical Fact Sheet, Noroviruses.
  2. Heymann D L, editor, Control of Communicable Disease Manual. 18th ed. American Public Health Association, 2004.
  3. Hawker J, Begg N, Blair I, Reintjes R, Weinberg J. Communicable Disease Control Handbook, Blackwell, 2005.
  4. Chadwick PR, Beards G, Brown D et al. Report of the Public Health Laboratory Service viral Gastro Enterits working goup: Management of hospital outbreaks of gastro-enteritis due to small round structured viruses, Journal of Hospital Infection (2002) 45: 1-10. Available online at:

© CM Kirwan 2006