Officials say more than thirty people are possibly infected with the highly contagious norovirus  on board a cruise ship sailing in California.

Princess Cruises spokeswoman Karen Candy says about 37 passengers on the Crown Princess reported being sick while the ship was in San Francisco on Monday, reports Fox News.

Candy said Tuesday that cruise officials believe the cause of the illness is the norovirus. That family of viruses is spread person-to-person and causes flu-like symptoms including nausea, vomiting, diarrhea and stomach cramping.

Candy says the ship's staff began intense disinfecting of surfaces, and all sick passengers are being encouraged to remain in their rooms.

The Crown Princess is on a seven-day California cruise that began in Los Angeles on Saturday and ends in Santa Barbara.