Cruises are sold as floating holidays, but they are also useful for understanding public health. Cruise ships are carefully designed places where many people live, eat, relax and move through the same shared spaces for days at a time. They show how easily illness can spread when people are packed into a single interconnected environment.

Think of a cruise ship as a temporary city at sea. It has restaurants, theatres, lifts, cabins, kitchens, water systems and indoor gathering spaces. That is great for convenience, but it also means that once an infection gets on board, it can move through the ship in ways that are hard to stop.

The Diamond Princess outbreak is perhaps the best-known example. During the 2020 COVID outbreak, 619 passengers and crew tested positive for the disease. Researchers found that the ship conditions made the novel coronavirus spread more easily. Their modelling suggested that public health measures, such as isolation and quarantine, prevented many more cases, but it also showed that an earlier response would have further limited the outbreak.

Norovirus (the so-called vomiting bug) is the infection most closely linked to cruise ships. In a review of previously published studies, researchers found 127 reports of norovirus outbreaks on cruise ships, with many linked to contaminated food, contaminated surfaces and person-to-person spread. A more recent report from the US also showed that norovirus can spread very rapidly from person to person on a........

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