The Center for Disease Control and Prevention on Monday was investigating the cause of a Norovirus outbreak on a Princess Cruises ship that caused 170 people to fall ill.

The Crown Princess arrived at the Port of Los Angeles on Sunday morning with 3,007 guests and 1,160 crew members aboard. It had been on a 28-day cruise from Hawaii to Tahiti when passengers started getting stomach flu.

“We can’t really pinpoint the specifics for the cause of the outbreak, it could be a multitude of things,” said Bernadette Burden, a spokeswoman for the Center for Disease Control and Prevention (CDC).

“When you have people gathering in small places the likelihood increases and there could be any number of potential causes that could have contributed – a sick passenger came aboard or a bad refrigerator,” she said.

This is not the first outbreak that the Santa Clarita-based cruise line has experienced this year.

In April, the same ship reported an outbreak in which 104 people contracted Norovirus on a seven-day cruise from San Francisco to Ensenada, Mexico. There were 3,161 passengers and 1,176 crew members aboard the Crown Princess at that time.

And in January, the Caribbean Princess based in Houston returned to port early after 170 passengers and crew members fell ill. According to media reports, some of them had symptoms that indicated Norovirus was also the culprit.

Norovirus is highly contagious and can be transferred through contaminated food and water or by touching infected surfaces, according to the CDC. The virus can cause nausea, stomach pains, diarrhea and vomiting.

After docking in San Pedro, the Crown Princess underwent a thorough cleaning that followed CDC disinfecting protocols. .

“We will continue our stringent cleaning and disinfecting protocols developed in conjunction with the CDC,” said Susan Lomax, spokeswoman for Princess Cruises.

A CDC health officer was aboard the ship Princess investigating the outbreak and working alongside the crew to eliminate the virus. The Crown Princess is scheduled to depart for a weeklong cruise to Mexico Sunday night.

The ship was due to arrive in Los Angeles Saturday, but took a detour to the Marquesas Islands last Monday when a crew member became sick and required surgery. The ailment was unrelated to the Norovirus outbreak, according to media reports.

Lomax said she does not expect that the Norovirus outbreak will affect the cruise line’s sales during the holidays.

“Our holiday cruises were mostly booked some time ago,” she said. “We generally find that consumers understand the importance of frequent hand washing during the flu season and the importance of limiting public interaction when sick to help contain the spread of the virus whether on land or sea. In fact, the chances of contracting Norovirus are greater on land than at sea on a cruise ship.”