A norovirus outbreak in Britain has caused closure of hospital wards to new patients in an effort to stop it from spreading between patients, officials said. Norovirus is highly contagious and can spread rapidly in closed environments such as hospitals, schools and nursing homes. It can be fatal in vulnerable people such as the very young and the elderly, officials said. The number of National Health Service beds closed due to the outbreak has doubled in a week, The Daily Telegraph reported Friday. Department of Health data showed the average number of beds closed per day rose from 720 to 1,545 between Dec. 11 and Dec. 18. Wards have been closed to new patients as part of steps to control the virus and allow deep cleaning, hospital officials said. Despite the recent rise, officials said levels of norovirus, also known as the winter vomiting bug, are below average for this time of year. \"The levels of norovirus seen this week are well within expected levels for this time of year,\" a Department of Health spokesman said.