SAN FRANCISCO - More than 300 passengers and crew aboard the Queen Elizabeth 2 were struck by a suspected stomach flu in recent days, US health officials said on Wednesday after the world famous cruise ship docked in San Francisco.

The US Centers for Disease Control and Prevention said it was notified by Cunard Line, owner of the Queen Elizabeth 2, on January 11 that some passengers had fallen ill with symptoms associated with norovirus, a virus responsible for gastroenteritis marked by stomach cramps, nausea, vomiting and diarrhea that last two to three days.

The number of sick passengers climbed to 276, from a total 1,652, and 28 of the ship's 1,002 crew also became ill, said Lisa Beaumier, public health analyst with the CDC.

While their symptoms were consistent with norovirus, health officials continue to study stool samples, Beaumier said.

US health officials boarded the Queen Elizabeth 2 in Acapulco, Mexico on Friday to investigate the outbreak. Its crew responded with increased cleaning and disinfection measures.

A Cunard Line spokesman in an e-mail to Reuters said all but six passengers sickened during the outbreak have recovered.

Norovirus is highly contagious and infection is common this time of year.

Nearly 400 passengers and crew were sickened by a common stomach virus on a Caribbean cruise aboard Royal Caribbean's Freedom of the Seas, the world's largest cruise ship, the company said in early December.