Norovirus is the most common cause of diarrhoea and vomiting in the UK. It is a highly infectious form of gastroenteritis that can strike very quickly. Although it is a very unpleasant illness, it usually clears up within two to three days and has no long-term effects.
Outbreaks of norovirus are common in hospitals, nursing homes, schools and on cruise ships.
Although infections can occur at any time of year, it is most common during the winter.
You can get norovirus several times because the virus is always changing, which means that your body cannot build up resistance to it.
Figures published by the Health Protection Agency (HPA), which tracks norovirus in England, showed that the number of laboratory confirmed cases so far this season had reached 3,046, compared with 1,669 for the same period last year.
However, experts say that many more people will have had the stomach bug because most people do not see their doctor when they get it. It has been estimated that for every confirmed case there are around 288 unreported cases, which means that around three quarters of a million people in England may have had norovirus already this winter.
The NHS estimates that between 600,000 and a million people in the UK get norovirus each year.
Wards shut, admissions cancelled
This year's winter vomiting bug has led to a number of ward closures and restrictions on hospital visiting.
Maidstone Hospital in Kent has shut three wards to new admissions and banned all visitors. Birmingham City Hospital has also closed its doors to visitors.
The George Eliot Hospital in Nuneaton has suspended all visiting until further notice after cases of norovirus were reported on several wards.
Visitors to Tunbridge Wells Hospital are being asked to help stop the spread of the tummy bug by only attending if absolutely necessary.
Three wards have been closed at Dumfries and Galloway Royal Infirmary after more than 90 patients and staff contracted the bug. Admissions have also been suspended at the nearby Castle Douglas Community Hospital because of the illness.
Two P&O cruise liners have undergone intensive cleaning after outbreaks of norovirus. Ten cases were confirmed on the Azura which arrived in Southampton at the weekend following an 11-night tour of Iberia. More than 400 passengers contracted the bug during a Baltic cruise on the Oriana which returned to the UK last Friday.
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