Eric Risberg / AP

The Sea Princess makes its way out to the Pacific Ocean, past the Golden Gate Bridge, on May 10. The ship was beginning a 10-day inside passage trip to Alaska.

The Alaska cruise ship Sea Princess is taking sea sickness to a new level. 

For the fourth time since mid-May, the ship's passengers have been sickened by norovirus, a gastrointestinal infection that causes diarrhea, vomiting and stomach pain. 

At the end of a 10-day sailing, the ship returned to San Francisco June 29 with 53 of its 2,196 passengers affected by the stomach bug, according to USA Today.

The number of cases wasn't high enough for the Centers of Disease Control and Prevention to consider the episode an official outbreak. The CDC issues an outbreak alert only when 3 percent of either passengers or crew members become ill; only 2.4 percent of Sea Princess' passengers were sickened.

A mid-May sailing of the Sea Princess led to 44 cases of norovirus. The next bout sickened 128 passengers on a tour that ended May 30. Despite increased efforts to disinfect the ship, another 142 passengers fell ill on the next outing, which ended June 9. The second and third episodes sickened enough passengers for the CDC to issue an alert. (See Norovirus strikes Alaska cruise ship three times in a month)

Princess Cruises held the ship in San Francisco on the 29th for another extensive cleaning, delaying the ship's departure for more than five hours. A spokesperson for the cruise line told USA Today that it was too soon to know whether any passengers on the ship's current tour were infected by norovirus.

In a statement Princess Cruises said, "We did experience an increased number of passengers with norovirus on three earlier Sea Princess cruises, but due to extra sanitation efforts and diligent communication with our passengers, no heightened occurrence of illness occurred on the June 9 cruise.

"We believe the reintroduction of the virus on the current cruise was inadvertently brought on board by embarking passengers, as we know norovirus is currently circulating widely throughout North America."

Jay Dempsey, a spokesperson for the CDC, told msnbc.com that officials have noted an increase in gastrointestinal infections during the summer cruising season. "We are seeing increased GI illness during this part of the year, but not in comparison to previous years," he said.

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Rebecca Ruiz is a senior editor at msnbc.com. Follow her on Twitter.