For the fourth time since mid-May, the Se Princess’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.