Hospital wards have been forced to close in an effort to contain the dangerous winter vomiting bug which is sweeping Britain . Cases of norovirus, which causes violent vomiting and diarrhoea, are on the rise and have been reported at a dozen hospitals in the past week. With further cold weather ahead, experts warn the illness could continue to spread. It is estimated that up to a million people each year are affected by the virus, which can spread rapidly, particularly in crowded environments such as hospitals, residential care homes and schools. Occasionally it can be fatal, especially if contracted by those who are already unwell. Hospitals in London, Warwick, Redditch, Doncaster, Winchester, Bedford and Newport in Wales have been forced to close wards. On the Isle of Wight, patients with symptoms of the bug have been warned to stay away from hospital, and are instead being treated in their homes. Carol Alstrom, the chief nurse at St Mary\'s Hospital on the island, said: \"It is vitally important for people to keep away from hospital and nursing or residential care homes if they have been unwell with vomiting or diarrhoea recently.\" Hugh Pennington, emeritus professor of microbiology at Aberdeen University, said that despite advances in medicine, treatments for norovirus have not improved in the last century. \"We have to fall back on traditional barrier nursing — isolation of patients and treating their symptoms, all things we could have done 100 years ago,\" he said. ‘Last straw\' \"For most people, norovirus is mild. But for the very elderly and the elderly with other health problems, it is the last straw for some of them.\" He added that most people do not bother their GP about the condition and so official figures are notoriously low. But for every one case in official statistics, it is likely there are many more. Norovirus, which is most prevalent in winter, is highly contagious and can live on tiles, curtains and carpets for weeks. There are also fears that a new strain is on the march. As well as vomiting and diarrhoea, other symptoms include a raised temperature, headaches, stomach cramps and aching limbs.