More than 120 passengers and crew members on a Princess Cruises ship that docked in San Francisco on Thursday were infected with a stomach virus while on their voyage, federal health officials said.

The Ruby Princess was on a 20-day round trip journey from San Francisco to Canada and Alaska when 102 passengers and 23 crew members were stricken by norovirus, the U.S. Centers for Disease Control and Prevention said.

Norovirus is highly contagious, often spread by food or on surfaces, particularly in crowded conditions. It is a short-lived illness for many people, but can be dangerous for people with underlying health conditions, young children and those aged 65 and older.