The number of people affected by an outbreak of the winter vomiting bug could have passed one million, new figures show.
The number of laboratory-confirmed cases of the norovirus has reached 3,538 in England and Wales, up from 3,046 last week and an 83% increase on the 1,934 cases at this time last year, the Health Protection Agency said.
But the HPA has said that for every reported case an estimated 288 go unreported, meaning there could be 1.01 million cases, up from just under 880,000 last week.
The HPA said that in the two weeks to December 23 there were 70 reported hospital outbreaks of the virus, compared to 61 in the previous fortnight, bringing the total of outbreaks for the season to 538.
The number of cases has risen earlier than expected this year, following an as-yet unexplained trend seen across Europe and other parts of the world.
John Harris, an HPA norovirus expert, said: "The number of laboratory confirmed cases has risen once again as it appears that we have seen the rise in cases that usually begins in January start a little earlier than we normally expect.
"Norovirus is very contagious, and very unpleasant. To help prevent spread of the disease, it's important that
people who believe they are unwell with the virus maintain good hand hygiene and stay away from hospitals, schools and care homes, as these closed environments are particularly prone to outbreaks which can cause severe disruption."
The bug has swept the country and has led to the closure of dozens of hospital wards.
It has also affected holidaymakers on two cruise ships.
Norovirus is highly contagious and can be transmitted through contact with an infected person or contaminated surfaces and objects.
It is known to spread rapidly in closed environments such as hospitals, schools and nursing homes.
Symptoms include sudden vomiting, diarrhoea, or both, a temperature, headache and stomach cramps. The bug usually goes away within a few days.
Although people can suffer from norovirus at any time of the year, activity increases in the winter months, with most cases seen between January and March.
The Royal Lancaster Infirmary was one of the latest hospitals affected. It closed all its wards to visitors for two days on December 19 in a bid to stem a "significant outbreak" that saw 140 patients and 20 staff affected by symptoms of the bug.
In a statement, the University Hospitals of Morecambe Bay NHS Trust (UHMBT), which runs the Lancashire hospital, said: "Despite every effort by the trust, the highly contagious winter bug has continued to spread, now affecting seven wards."
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