More than 170 passengers and crew fell ill with norovirus on a cruise ship that docked in Los Angeles, California, on Sunday after a nearly month-long trip, according to the Centers for Disease Control and Prevention.

The total number provided by the CDC includes those who were sick at some point during the Crown Princess trip: 158 of the 3,009 total passengers, as well as 14 of the 1,160 total crew. Symptoms of norovirus include vomiting and diarrhea.

Considering the number of people who go on cruises every year, the incidents of illness on ships are relatively rare. The cruise industry reportedly serves more than 20 million people each year.

Unfortunately for the industry, however, it is associated with norovirus because health officials are required by law to track onboard outbreaks. If hotels or resorts were required to report the disease, public perception of these location would likely be damaged.

However, there have been four outbreaks on Princess Cruises’ Crown Princess ship since 2012.

The last known outbreak was on the same ship in April, when 94 passengers and 23 crew came down with norovirus symptoms. That trip was a seven-day voyage that started in Los Angeles.

Norovirus is very common and very contagious; people can become infected if they eat food or drink liquids that are contaminated by handlers, or if they touch contaminated surfaces and then touch their mouths. Direct contact with another person who is sick can also spread the disease.

In response to the outbreak, Princess Cruises are increasing cleaning and disinfection procedures on board, according to the CDC. Onboard passengers will also be notified, and encouraged to report if they fall ill. In addition, the cruise line will alert passengers who are set to board the Crown Princess on Sunday about the outbreak.

A CDC official will board the ship Sunday to assess the outbreak and Princess Cruises’ response.

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