Nairobi, May 12 -- 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 outb...