Twenty-five crew members and 152 of the 2,144 passengers onboard the May 15 to May 25, 2023 voyage of the Celebrity Summit reported norovirus symptoms, according to a notice issued by the Centers for Disease Control and Prevention.

The primary cause of mass illness on cruise ships throughout 2023 was Norovirus, often referred to as the "stomach flu." According to data from the CDC's Vessel Sanitation Program, 2023 saw one of the highest spikes in norovirus outbreaks in over a decade, with 13 major incidents reported on lines including Royal Caribbean, Celebrity, and Princess Cruises. Norovirus is an extremely contagious virus that causes acute gastroenteritis (vomiting and diarrhea). It spreads rapidly in closed environments like cruise ships through contaminated food, water, or surfaces, and through person-to-person contact. In one notable 2023 case, over 170 passengers on a single sailing were affected. Cruise lines responded by implementing "Level 3" cleaning protocols, which involve deep disinfection of high-touch areas and the suspension of self-service buffets. While norovirus is rarely fatal, it can ruin a vacation for hundreds of people simultaneously, leading the industry to double down on hand-washing campaigns and high-tech electrostatic spraying between every voyage.