The Centre for Health Protection on Friday urged the public to cook food thoroughly, particularly shellfish, to avoid contracting norovirus in the wake of a spike in food-poisoning cases.

That call came as it recorded 45 incidences of food poisoning between January 18 and February 12 – three times the number of similar cases reported during the same period last year.

About 87 percent of the latest food-poisoning cases involved norovirus, and all of those battling the illness had eaten raw oysters within 12 to 48 hours, the incubation period, before symptoms showed.

Albert Au, who heads the CHP's communicable disease branch, told RTHK that norovirus is highly contagious and that people can easily contract the disease by consuming raw oysters.

"Raw oysters are a high-risk food. Why do I say that? Because oysters feed by filtering a large amount of seawater," he said.

"If the seawater, or water at an oyster farm is contaminated, such as with the excrement of people who are infected with norovirus, then pathogens can easily accumulate in the oysters, particularly in their digestive systems."

People of all age groups can catch norovirus, Au said, and the illness can easily become serious and require hospitalisation for those who are vulnerable, especially infants and the elderly, or cause dehydration.

The correlation between acute gastroenteritis and norovirus is also high, he pointed out.

Care homes and schools have logged 38 acute gastroenteritis outbreaks so far this year, affecting 294 people, with norovirus being the cause in three quarters of the cases, Au added.

People should wash their hands before eating and opt for reputable eateries.