More than 100 passengers and roughly 23 crew members aboard a Princess Cruises ship have become ill in a suspected norovirus outbreak, the Centers for Disease Control and Prevention said. It is the third such outbreak reported on one of the company’s vessels this year.

The Ruby Princess departed San Francisco on June 12 for a voyage to Alaska and Canada, with its return scheduled for July 2. CDC officials were notified of the outbreak more than two weeks into the trip, after the number of reported illnesses met the agency’s outbreak threshold of at least 3% of passengers. The ship was carrying 3,032 passengers and 1,144 crew members, according to the CDC.

The vessel was docked in San Francisco on Thursday for cleaning and disinfection.

Norovirus is a highly contagious virus that can cause vomiting and diarrhea. It commonly spreads when microscopic particles of feces or vomit enter the mouth, including through contact with contaminated surfaces or objects followed by hand-to-mouth contact.

A representative for Princess Cruises did not immediately respond to a request for comment about what may have caused the outbreak.

Public health officials advise frequent handwashing with soap and water, especially after using the bathroom and before eating, as one of the most effective ways to reduce the spread of germs.

Cruise ships can be particularly vulnerable to the rapid spread of illness because passengers and crew live and gather in close quarters. The CDC has recorded six other norovirus outbreaks on cruise ships so far this year.

On the Ruby Princess, passengers and crew members who became sick have been isolated while the ship undergoes disinfection procedures.