As the weather turns chilly, people traditionally like to dine on hotpots, but are being warned to make sure their food is properly cooked to prevent stomach infections. Taking seafood out of the boiling broth before it is thoroughly cooked means it could still be carrying norovirus, a common cause of food poisoning, the Centre for Health Protection said yesterday. Children and the elderly should be particularly careful, it said. The advice from consultant Thomas Tsang Ho-fai came after Tokyo announced on Tuesday that the number of norovirus cases in Japan had reached a 25-year high. Dr Tsang said the virus peaked in Hong Kong at the beginning of the month, as was usual. Oysters, in particular, and other raw seafood are a common source of the virus. 'Bivalves should be cooked thoroughly,' he said. 'Otherwise, the risk of contracting norovirus will be much higher.' There had been 20 outbreaks in Hong Kong so far this month affecting 204 people, compared to 12 cases affecting 102 people in November. Most of the outbreaks were in homes for the elderly. Dr Tsang said it was not known whether the outbreak in Japan was caused by a mutated norovirus. Hong Kong experienced an outbreak caused by a new variant from May to August, Dr Tsang said. The virus, which causes severe vomiting and diarrhoea, is not life threatening, but is more easily transmitted than Sars. Dr Tsang advised the public to pay close attention to personal and household hygiene, saying any vomit should be swiftly cleaned up by soaking the affected area with diluted bleach for at least 45 minutes.