More than 300 people are thought to have caught a highly infectious virus on the Queen Elizabeth 2 cruise ship.
The Norovirus, a vomiting and diarrhoea bug, is common in cruise ships, hospitals and prisons.
After the Southampton-based ship docked in San Francisco, US officials said 276 passengers and 28 crew were hit, although just six were still sick.
A Cunard spokesman said the virus had been largely eradicated after first flaring up on 8 January.
All those who caught the gastrointestinal virus will have suffered symptoms for a few days before recovering. There is usually no lasting damage.
Health officials said this was an unusually large outbreak, with 17% of those on the luxury ship affected.
Confined to cabin
The Cunard spokesman said: "The whole cleansing regime has been upgraded. People have been advised on measures they should take.
"We have encouraged handwashing. Where people display symptoms they are kept confined in their cabin until the symptoms subside."
The ship sailed from Southampton on 2 January, before changing most of its passengers in New York on 8 January and sailing down the US's east coast.
It is thought a passenger brought the disease on in New York, the spokesman said.
Officials from the US Centers for Disease Control boarded the ship in Acapulco, Mexico.
The ship is now on its way to Hawaii.