SAN DIEGO -- Eating raw oysters during the holiday season might be fashionable, but they're much safer if they're cooked, the county public health officer said Wednesday.

Oysters sometimes contain the bacteria Vibrio parahaemolyticus, which can cause gastrointestinal illness, Dr. Wilma Wooten said.

In healthy adults, the sickness can be minor. But older people and those with weakened immune systems could be at risk for more serious problems, Wooten said.

In serving oysters over the holidays, Wooten recommends:

Making sure the oysters come from a safe and approved source

Asking where the oysters were harvested

Purchasing oysters with closed shells

Keeping the oysters properly refrigerated to prevent bacterial growth

Washing hands after handling oysters

Boiling oysters until their shells open, then an additional three to five minutes

"Thoroughly cooked oysters reduce the risk of illness from bacteria," Wooten said.