What the winter bug does to you

The bug is commonly known as the small round structured virus in Britain, but its more official title is the Norwalk-like virus, after the town in Ohio, in the United States, where it was first identified.

It causes unpleasant symptoms such as projectile vomiting and diarrhoea and fever for between 24 and 48 hours. "We are not talking about feeling a bit dicky and chucking up in the toilet bowl. I am told that people can vomit straight out for about a yard," said Dr John Cowden, consultant epidemiologist at the Scottish Centre for Infection and Environmental Health in Glasgow.

The virus is airborne and can pass from person to person with ease, for example through coughing and vomiting.

It is often found in hospitals, schools, or places such as cruise ships. Restaurants can also be affected, as the bug can be food-borne.

The bug is rarely life-threatening, with almost everybody affected recovering completely.

However, it costs the NHS huge sums of money every year because of hospital outbreaks.

Health officials say the number of reported cases is the tip of the iceberg, as many people suffer the illness but do not contact their doctor.

Research conducted by Dr Cowden has shown that for every case recorded in official statistics, there are about 1,500 cases which are unreported.

There were 1,951 laboratory reports of the virus in England and Wales in 2000 compared with 208 in the same year in Scotland. Scotland reported 328 official cases in 2001.

The Department of Health estimates that the bug affects up to 1% of the British population every year.