TOKYO—After decades as a primary-care doctor in Elizabethton, Tenn., Arnold Hopland thought he knew something about preventing infections. And the way they were doing a virus quarantine on the Diamond Princess cruise ship didn’t look right.

Dr. Hopland, a passenger, counted as many as 10 interactions a day with staff coming to deliver meals, toiletries and chocolate to keep up spirits. Passengers were drying laundry on their balconies without wearing face masks. Some leaned over to talk to their next-door neighbors.

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