More than 3,500 cases of the norovirus winter vomiting bug in England and Wales have been confirmed in laboratories this season - but the true number could be over one million.
The 3,538 figure - for July to mid-December - is 83% higher than the same period in 2011 where there were 1,934.
The number of confirmed cases have risen by about 500 from last week's 3,046, said the Health Protection Agency (HPA) .
But the agency added that for every reported case an estimated 288 go unreported, meaning there could be 1.01 million cases, a rise from just under 880,000 last week.
It said in the two weeks to 23 December 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, the HPA went on.
It has not yet been determined why this has been the case, but it is a trend that has been seen across Europe and other parts of the world.
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.
John Harris, an expert in norovirus from the HPA 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."