• Family: Caliciviridae
• Genome: Non‑enveloped, positive‑sense ssRNA
• Size: ~40 nm
• Genogroups: GI and GII (most outbreaks); GII.4 historically dominant, but GII.17 also rising (UKHSA 2025–26 data)
• Reservoir: Humans

Norovirus, also known as the winter vomiting bug, is one of the most common causes of gastroenteritis in the UK according to NHS England. It is a genus that encompasses a range of non encapsulated RNA virus species. The CDC (Centre for Disease Prevention and Control) estimates that 1 in 5 cases of infectious gastroenteritis are caused by norovirus,

• Leading cause of acute gastroenteritis worldwide (≈685 million cases/year)
• Extremely infectious: infectious dose as low as 18 viral particles
• Seasonality: “Winter vomiting bug” — peaks Nov–Apr in northern hemisphere
• Outbreak settings:
  • Care homes (most common)
  • Hospitals
  • Schools, nurseries
  • Cruise ships
  • Restaurants/catered events
• Attack rates: Up to 50% in institutional outbreak

Symptoms

• Develop within 15 - 50 hours of infection (quoted from the European Centre of Disease Control and Prevention), with patients experiencing nausea, vomiting, and diarrhoea, which may be accompanied by headaches, low-grade fevers, and myalgia.
• The majority of patients experience both vomiting and diarrhoea
• 10 - 22% reported a headache and up to 47% of patients reported fevers (Clinical Microbiology Reviews, Norovirus, January 2015)

Transmission

• Faecal-oral route, with the virus becoming aerosolized when the patient vomits or when a toilet containing infected bodily fluids (vomit or faeces) is flushed.
• Viral particles are transmitted directly to surrounding potential hosts and to surrounding surfaces, from which they can be transmitted by cross-contamination (for example a patient visitor touching the patient environment, then eating without first washing their hands).
• Infection may also be transmitted from an infected individual in the process of direct physical contact, or contact with food preparation.
• The virus enters the cell via host receptor-mediated endocytosis and replicates in the small intestine. Norovirus is highly contagious, the European Centre of Disease Control and Prevention estimates that only 10 - 100 viral particles are required to cause an active infection.