More than 750,000 people are thought to have been struck down by an outbreak of norovirus in the UK - an increase of 72%.
The Health Protection Agency (HPA) says there are likely to be a further 288 unreported sufferers for every one of the 2,630 confirmed cases of norovirus so far this season.
A spokesman for the HPA said the figures meant 757,440 people could be affected by the stomach bug, which causes vomiting and diarrhoea.
"Laboratory confirmed reports represent only a small proportion of the actual amount of norovirus activity in the community, because the vast majority of affected people do not access health care services as a result of their illness," he said.
Among the reported cases are passengers on board a liner in the Baltic, who have been confined to their rooms and warned not to leave the ship.
Three hundred of the 1,843 passengers on the Oriana were hit by the highly contagious bug, according to the Daily Telegraph.
A spokeswoman for P&O Cruises said six people on board had active symptoms, but she was unable to confirm how many people had been affected in total.
The outbreak occurred after the Oriana departed from Southampton on a 10-day cruise last Tuesday.
A P&O spokeswoman said today: "The safety and comfort of passengers and crew is always our number one priority.
"As is currently standard procedure across our fleet, all the ship's passengers were provided with a precautionary health notice advising of widespread norovirus activity and the health measures to avoid contraction and spread, both on board and whilst ashore."
She added steps had been taken to minimise the spread to other passengers and that any medical charges arising will be met by the company.
A spokesman for the HPA said there was no obvious reason why there are more cases of norovirus this year than last.