More than 170 passengers and crew members on board a Princess Cruises cruise ship that was docked in San Pedro Sunday have fallen ill following an outbreak of Norovirus, according to the Centers for Disease Control and Prevention.

The Crown Princess ship was sailing from Tahiti to Los Angeles on a month-long cruise when passengers began exhibiting symptoms of the gastrointestinal illness, according to a statement from Princess Cruises that was sent on Sunday.

“Over the last few days, the ship began seeing an increased number” of the cases, the statement read.

About 20 people currently had active cases of the illness, which causes vomiting and diarrhea.

Princess Cruises has taken active steps to combat the virus, including increased cleanings and disinfection procedures, and encouraging hand hygiene and case reporting, the CDC Vessel Sanitation Program reported.

Other steps would be taken once passengers disembarked from the ship to prevent the virus from spreading.