SAN FRANCISCO (KCBS) — Thirty-seven passengers aboard the Crown Princess cruise ship, that docked in San Francisco on Monday, have contracted the stomach flu, a spokeswoman for Princess Cruises said.

Princess Cruises spokeswoman Karen Kandy said that passengers had reported to the infirmary with symptoms of a mild gastrointestinal intestinal illness caused by the Norovirus, which is common but extremely contagious.

She said the ship’s crew has stepped up sanitation procedures on the ship to prevent further spread and that the more than 3,000 passengers on the cruise are being reminded of what they can do to avoid exposure—such as washing their hands frequently.

None of the stricken passengers left the ship after docking, opting to continue sailing on the seven-day coastal cruise that departed from Los Angeles, according to Candy.

The Crown Princess docked in San Francisco Monday on its way to Santa Barbara. The ship has since left the Bay Area and is somewhere off the Central California coast.