More than 100 hospital wards across the UK have been closed to new patients in a bid to slow the spread of the vomiting bug norovirus.
Doctors estimate that more than 100,000 people a week are catching the bug.
One hospital trust has cancelled non-urgent operations and others are trying to discourage visitors in case they bring in the virus.
People struck down by norovirus have been urged not to go back to work until the symptoms have fully disappeared.
Reported cases of the illness from early December are at a five-year-high, but the real figure is likely to be much greater as most sufferers do not seek medical attention.
Although affected wards are still functioning, they are not admitting new patients.
Wards have been closed around Scotland, including five at Victoria Infirmary in Glasgow.
Fifteen wards in Greater Manchester are closed to new admissions.
Eleven of those are at the Royal Oldham Hospital, where more than a quarter of the total are shut.
A spokesman there told the BBC News website that the closures meant 26 beds were unable to be used on Thursday. That figure fell from 40 on Wednesday.
The Worcestershire Acute Hospitals Trust has cancelled all non-urgent operations until 9 January because of what it calls the "unrelenting pressure" caused by the virus.
Some hospitals, including two in Bristol and one in Bedford, have told people not to visit if they have been sick or if their visit is not essential.
A spokeswoman for the Department of Health told the BBC News website that ward closures were implemented to contain the virus and stop it spreading to new, potentially vulnerable patients.
Norovirus - also known as winter vomiting disease - spreads by contact with an infected person, through contaminated food or water or by contact with contaminated surfaces or objects.
It is not normally dangerous, but the very young and very old are most at risk of complications from dehydration.
Outbreaks are common in hospitals, nursing homes, schools and cruise ships.
The HPA estimate that in epidemic seasons, noroviruses may cost the NHS in excess of £100m per year because of ward closures.
And NHS Direct said calls about vomiting and abdominal pain had been the number two reason for people calling over the Christmas period.
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