SAN FRANCISCO — 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.

Symptoms include sudden onset of vomiting, diarrhea and stomach pain that can last three days, according to the CDC.

The outbreak on the Ruby Princess, which set sail on June 12, was reported to the CDC on Saturday, officials said, adding that not all the infected people were sick at the same time or when the ship arrived or left port.

There were 3,032 passengers and 1,144 crew members on the Ruby Princess voyage, the CDC said.

Princess Cruises said in a statement that the ship’s crew responded promptly and implemented “enhanced sanitation protocols across the ship.”

The ship will be thoroughly cleaned and disinfected before departing on its next trip later Thursday, the company said.

So far this year, there have been seven illness outbreaks reported on cruise ships in the CDC’s jurisdiction, mostly from norovirus, health officials said.

Most norovirus outbreaks occur when people who are already infected spread the virus to others by direct means, such as through sharing food or utensils. Outbreaks can also be spread through food, water or contaminated surfaces.

Washing hands frequently is key to preventing a norovirus infection, especially after using the bathroom or before eating and drinking. It is best to use soap and water, scrubbing and washing for at least 20 seconds. Hand sanitizer alone doesn’t work well against norovirus, the CDC said.