More than 600,000 people are believed to have been struck down in a surge in a winter vomiting bug this winter, the Health Protection Agency (HPA) has said.
Laboratory tests have revealed a 64% rise in the number of people with the norovirus, with 2,313 confirmed cases on December 2, compared with 1,412 at the same point last year.
But the HPA estimates there are 288 unreported cases for each confirmed one, which means 666,144 people are likely to have contracted the illness.
The agency said it was unsure why more people were suffering with the virus but added that the norovirus season was always "unpredictable".
Despite the increase in laboratory cases, figures show a slight fall in the number of hospital outbreaks of norovirus, from 53 in the two weeks to November 25 to 40 in the two weeks to December 2.
Dr Bob Adak, head of the gastrointestinal diseases department at the HPA, said: "The norovirus season is always completely unpredictable as it peaks and falls over several months - usually October to April.
"However, one thing we do know is that every year we will see a large amount of norovirus activity because it is highly contagious.
"The figures for the last two weeks show that there has been a reduction in the numbers of outbreaks in hospitals although there are clearly a lot of other people becoming unwell as we can see from the number of lab reports.
"We would like to remind people to avoid visiting friends or relatives in hospital or care homes if they have symptoms consistent with a norovirus infection as it can lead to ward closures and severe disruption."
Last week the HPA reported an outbreak of norovirus had led to the closure of 45 hospital wards across the previous fortnight.
The virus is highly contagious and can be transmitted by contact with an infected person, by consuming contaminated food or water or by contact with contaminated surfaces or objects.
It spreads rapidly in closed environments such as hospitals, schools, nursing and residential homes, the HPA said.
Symptoms include a sudden onset of vomiting and diarrhoea, while some people may have a temperature, headache and stomach cramps.
The illness usually resolves in one or two days and there are no long-term effects, the HPA said.
Dr Adak added: "Norovirus is a short-lived unpleasant infection but most people will fully recover in a couple of days.
"It is important to remain hydrated as you will be losing a lot of fluids due to the symptoms. Over-the-counter medicines can also be useful in reducing headaches and other aches and pains."