Cruise company Fred Olsen has confirmed there has been an outbreak of "a gastroenteritis-type illness" on board a cruise ship which left Belfast last week.
The Boudicca left Northern Ireland on Monday 23 September on a 10-night 'Scandinavian Cities Cruise'.
The company has said a number of guests "have suffered from this illness".
It is understood 72 passengers out of just over 760 on board have been infected.
The ship is to return to Belfast on Thursday and a sanitisation and cleaning programme will be carried out.
The company has contacted guests booked on the next scheduled cruise on the ship, a 12-night Madeira and Canaries excursion, to tell them what happened.
Symptoms of the illness include a sudden onset of vomiting and/or diarrhoea; some people may have a temperature, headache and stomach cramps. The sickness usually lasts one or two days.
"In such instances, every precaution is taken on board to ensure the swift containment of the illness, and a number of public areas, including all public toilets and the self-service buffet, are closed," the company said.
"Frequent cleaning of all areas of the ship is carried out, and any guest showing symptoms of the virus is asked to remain in their cabin, with complimentary room service and in-cabin entertainment, until they are considered to be symptom-free by the ship's doctor, before being able to join the rest of the ship.
"At Fred Olsen Cruise Lines, the health, safety and well-being of all our guests and crew is paramount, and we believe that our systems for preventing the spread of illness on board our ships are amongst the best within the industry."