Health officials have issued warnings about the serious threat posed by norovirus as the winter diarrhoea and vomiting bug forced the closure of hospital beds.
Maurice Madeo, deputy director of infection prevention and control at Doncaster Royal Infirmary, where wards were closed to new admissions and poorly patients segregated, said the condition could be fatal.
Warning people of the importance of thoroughly washing their hands, he said: "We can't be too careful about limiting the spread of all infections. An infection on top of another medical condition can prove fatal to sick, frail patients. So we all need to be mindful of the need for best practice in personal hygiene."
Medical bosses at hospitals across the country are working to prevent the condition spreading and to reopen wards that were closed on Saturday.
Eighty-two beds were closed on two wards and bays at the 500-bed Northwick Park in Harrow after between at least 14 and 16 patients developed the illness. Hospital spokesman Mark Purcell said one ward was scheduled to reopen after being deep cleaned.
Doncaster and Bassetlaw Hospitals NHS Trust said wards were also closed to new patients at Montagu Hospital in South Yorkshire because of the seasonal sickness.
Two wards were also closed at Warwick Hospital because of patients suffering from diarrhoea and vomiting but a spokeswoman said test results for Norovirus were not back.
The Richard Wells Ward was closed at Bedford Hospital was also closed on Saturday to stop the spread of the contagious viral gastroenteritis among patients, visitors and staff. Visiting to the ward was restricted to one visitor per patient per day.
The first sign of Norovirus is usually a sudden sick feeling followed by forceful vomiting and watery diarrhoea. Other symptoms include a raised temperature, headaches, stomach cramps and aching limbs.
The Royal College of GPs say the virus has not been a major problem so far this year - although outbreaks can occur very quickly, as the complaint is extremely infectious.