Norovirus may have affected 100,000 over Christmas, says health agency A rise in laboratory confirmed cases of the contagious norovirus is expected in the next few weeks, says the HPA. Photograph: Christopher Furlong/Getty Images The winter vomiting bug affected about 100,000 people over Christmas with the number of confirmed cases 72% higher than this time last year, Health Protection Agency figures indicate. The dramatic rise was attributed in part to an early outbreak of the norovirus-caused illness, which closed dozens of hospital wards as it struck across Britain. Laboratory-confirmed incidents of norovirus amounted to 3,877 cases recorded in England and Wales, a rise from the 2,255 tally of last year. The Health Protection Agency (HPA) said that for every reported case an estimated 288 were not flagged up, meaning that about 1.12 million people could have contracted the illness this season. The number of reported cases dropped slightly during the Christmas week but this was in keeping with declines at the same time in previous seasons, the HPA said. John Harris, an HPA expert in the norovirus, said: "As we have seen in previous years there has been a dip in the number of confirmed laboratory reports owing to the Christmas and New Year period. However, in line with other norovirus seasons we will expect to see an increase in the number of laboratory reports in the next few weeks. Norovirus is very contagious, and anyone who has had it knows it is very unpleasant. If you think you may have the illness then it's important to maintain good hand-hygiene to help prevent it spreading. During the two weeks up to 30 December, 29 hospital outbreaks were reported, compared with 70 in the previous fortnight, bringing the season's total to 590. Cases of norovirus started to increase earlier than expected across Europe this winter. Experts have been unable to determine why or predict whether numbers will continue to rise at the same rate. The bug, which affected holidaymakers on two cruise ships before Christmas, can be transmitted through contact with an infected person or contaminated surfaces. It is known to spread rapidly in closely populated environments such as hospitals, schools and nursing homes. Symptoms, which usually last a few days, include sudden vomiting, diarrhoea, or both, a temperature, headache and stomach cramps. Although people can suffer from norovirus at any time of the year, cases increase in winter, with most recorded between January and March.