A norovirus outbreak aboard the luxury cruise ship Ruby Princess has left more than 100 passengers and crew members ill during a 20-day voyage to Alaska.

The vessel, operated by Princess Cruises, docked in San Francisco on Thursday after 102 passengers and 23 crew members reported symptoms including vomiting and diarrhoea, according to the US Centers for Disease Control and Prevention (CDC).

Norovirus, often described as a stomach bug, causes acute gastrointestinal illness and spreads easily through close contact, contaminated surfaces, food, or drink. Health experts warn that infected people can continue to spread the virus for up to two weeks or more after their symptoms have stopped.

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The outbreak on the Ruby Princess was first reported on June 28. According to the available data, just over 3% of passengers and around 2% of crew members fell ill.

The ship had departed from San Francisco on June 12, with stops in Canada and Alaska, before returning to California this week.

Princess Cruises said the crew immediately increased cleaning and disinfection measures, while stool samples were collected from affected passengers and crew for laboratory testing. Those showing symptoms were isolated, and the company remained in contact with the CDC’s Vessel Sanitation Program in an effort to contain the outbreak.

“Our crew responded immediately by implementing enhanced hygiene protocols throughout the ship, and cases have since declined and remain at low levels,” the company said in a statement.

Princess Cruises added that the Ruby Princess is undergoing extensive cleaning and disinfection before its next voyage.

Cruise ship outbreaks this year

Since the beginning of the year, the CDC has recorded seven gastrointestinal illness outbreaks on cruise ships. Three involved Princess Cruises vessels and were linked to norovirus, with previous incidents reported on the Star Princess and the Caribbean Princess during voyages in March and between late April and May 11.

Two further norovirus outbreaks were reported this year by Lindblad Expeditions aboard the National Geographic Sea Bird, while two E. coli outbreaks were also recorded on other cruise ships.

How norovirus spreads and how to prevent it

The CDC recommends frequent handwashing, thoroughly cooking shellfish, washing fruit and vegetables, cleaning and disinfecting surfaces, washing clothes in hot water, and staying isolated for two days after symptoms have stopped.