“Get off that ship — there are a lot better places to isolate people,” Purdue University’s Qingyan Chen, an expert on ventilation during virus outbreaks, told BuzzFeed News. “In ships, you cannot filter the air well enough to stop viruses.”

Health experts have warned about the potential for cruise ship outbreaks for years. A ship’s ventilation system, which relies on recirculated air filtered by medium-strength air filters, is an efficient way of spreading virus particles from room to room aboard a ship, said Chen. In a 2015 study, he and his colleagues looked at the spread of flu aboard cruise ships, finding that one infected person would typically lead to more than 40 cases a week later on a 2,000 passenger cruise, with transmission occurring through the ventilation system. In contrast, on land, the coronavirus seems to have a reproductive rate of two new cases per infected person, which would lead to three new cases in that time.

A 2018 CDC study of two Alaskan cruise ships that suffered flu outbreaks found that 83% of 410 passengers on the two ships were infected with some respiratory illness within the second week of the cruises, with how soon passengers boarded the ship presenting the biggest determinant of their risk.