Norovirus infections are mostly unreported

Norovirus is also known as 'winter vomiting sickness' because of its violently unpleasant though seldom fatal symptoms and is commonly associated with oysters that have been contaminated with sewage.

It is probably the most common viral infection, said Dr David Brown of the Health Protection Agency's Virus Reference Department. However, Hepatitis A is also a foodborne viral problem in the UK and can pose more serious health problems.

Addressing the Food Standards Agency's Advisory Committee on the Microbiological Safety of Food, Brown said: "The national picture does not give a true indication of the burden ... These mask 100,000 foodborne infections each year, of which 70% may be Norovirus."

Norovirus is commonly passed on by infected food handlers, and environmental surface testing of, for example, fridge door handles within catering outlets may prove useful in investigating outbreaks, he added.

While it had been thought that individuals could not build up immunity to infection, "understanding has moved on", said Brown.

He said that research now indicated that it might be possible to inoculate some people against infection. "There is a vaccine trial going on in the US," he said, but added this was "some years away from being relevant to us".