Almost three million people have been affected by the norovirus stomach bug so far this winter, figures suggest.
Surveillance from the Health Protection Agency shows cases in England and Wales are double those seen last year.
Doctors advise people to stay at home for 48 hours after symptoms have gone to cut the risk of the bug spreading.
The HPA said the norovirus season began unusually early. For every one of the 1,922 reported cases, it is estimated another 1,500 have been unreported.
These cases will have been in people who did not visit the doctor.
This equates to around 2.8 million people affected so far this winter and the virus is still circulating.
Norovirus - also known as winter vomiting disease - is the most common cause of infectious gastroenteritis in the UK.
Hospitals have been affected by outbreaks with many wards around the country having to close to new admissions to prevent the spread of the illness.
Easily spread
Onset is very sudden with vomiting and diarrhoea.
Some people may also feel feverish.
Illness can occur at any age because immunity to it is not long-lasting.
It is not normally dangerous but the very young and very old are most at risk of complications from dehydration.
The HPA said they had expected a higher number of recorded cases as methods used for detecting norovirus in the laboratory had improved.
But they advised people to practise good hygiene including hand washing and disinfecting contaminated surfaces if anyone has become ill.
Food preparation should also be avoided until 48 hours after symptoms have disappeared.
There is no specific treatment for norovirus other than letting the illness run its course but it is important to drink plenty of fluids to prevent dehydration, especially in the very young or elderly.
Professor Steve Field, chair of the Royal College of GPs, said the advice issued last week on staying at home if you become ill and not rushing back to work was helping to slow the spread of the illness.
"Anecdotally the pressure seems to be coming off - we're delighted that people are following the advice and taking the pressure off the health service."
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