A Princess Cruises ship that sailed from Los Angeles was docked Thursday in San Diego after an outbreak of stomach flu among 104 passengers and crew.

The Valencia cruise line said the culprit was a Novovirus, a highly contagious bug that causes inflammation of the intestine, leading to vomiting, nausea and diarrhea.

By late Thursday morning only 15 people still had symptoms, but 50 remained in their cabins until they became non-contagious. Most of the passengers were spending the day visiting San Diego.

The vessel has 3,141 people aboard and is on a seven-day cruise with destinations from Ensenada, Mexico to San Francisco. Princess Cruises issued a statement that it was working to get the outbreak under control.

“We are continuing to implement enhanced sanitation procedures to interrupt the person-to-person spread of this illness,” the company said. The sanitation measures include cleaning of high-touch surfaces like railings, door handles and elevator buttons; encouraging passengers to wash their hands; and isolating sick passengers until non-contagious.

This isn’t the first outbreak for Princess. In January, the Caribbean Princess based in Houston returned to port early after 170 passengers and crew members fell ill. According to media reports, some of them had symptoms that would indicate Novovirus also was the culprit.

Princess Cruises is a unit of Carnival Corp. & plc in London.