“The ship is an enabling environment for an illness such as Covid-19 to spread,” said Jean-Paul Rodrigue, professor of transport geography at Hofstra University in New York. “Air circulation there is worse than on an aeroplane.”

“A cruise ship is an almost ideal environment to enhance the transmission of a virus, whether norovirus, coronavirus, or flu, from person to person,” said William Schaffner, an infectious disease specialist at Vanderbilt University.

On planes, air is cycled through the cabin, with a portion of it coming from outside. The air from outside would be naturally sterile, given the high altitude. Air filters used in planes are of the same quality as those in operating rooms.

The US Centres for Disease Control and Prevention (CDC) has risk classifications for plane seating – the risks are higher for people who are within two seats of an infected passenger.

But inside a cruise ship, passengers and crew are in much closer quarters, with the situation more dire for crew. Said Schaffner: “Staff are confined to dormitories, and they interact constantly to keep the ship functioning.

“They have prolonged face-to-face contact often within enclosed spaces, and pass each other often in close corridors. This is an environment where the transmission of viruses is very ready to take place.”

Some doctors suggest that having a room with a window or a balcony could improve air flow and therefore reduce the risk for some passengers.

Michael Osterholm, director of the Centre for Infectious Disease Research and Policy at the University of Minnesota, said: “We know that if you want to enhance the transmission of a virus like this coronavirus, or the influenza virus, put them inside a cruise ship where the air exchanges are compromised, where there are people basically sharing the air of others with themselves.”

Experts acknowledged that the authorities in Yokohama had no easy choice when faced with the ethics of confining all guests on board the Diamond Princess.

They may have believed that given the cruise industry’s past experience in dealing with major outbreaks of norovirus – about 10 annually in the past five years, according to the US CDC – there would be procedures to sanitise ships when outbreaks occur.

But vessel sanitation procedures often involve completely disembarking passengers from a ship while it is cleaned.

Rodrigue at Hofstra University said: “In Japan, [the authorities] thought the quarantine on board would be fine and they were caught by surprise. They’ve realised that a cruise ship is pretty much the worst environment to be stuck in.”

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Researchers looking at past norovirus outbreaks have found cruise ship outbreaks were an early indicator of emerging variants.

Marian Koopmans, head of the virology department at Erasmus Medical Centre in Rotterdam, told US news site Vox: “For a virus that likes to spread in groups of people, cruise ships are notorious.”

While the passengers who have disembarked from the Diamond Princess have already passed a two-week quarantine period, the health authorities in the US, Canada, Australia, South Korea and Hong Kong are not giving them the all-clear.

Officials have said returning passengers will be required to spend another 14-day period in quarantine, in some cases on military bases, to be sure they do not pose a risk to the public.

Japanese health minister Katsunobu Kato encouraged passengers departing from the Diamond Princess to monitor their health and limit their time in public.

In a widely circulated video this week, Japanese infectious disease specialist Kentaro Iwata lambasted the infection-control procedures aboard the Diamond Princess. Iwata said the ship was “completely inadequate in terms of infection control”, and that on board there was no differentiation between areas where people were infected and areas for people who did not have the virus.

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Stanley Deresinski, an infectious disease specialist at Stanford University, said: “Given the uncertainties regarding the transmission of the virus, the optimal approach would have been to take the passengers off the ship and place them in quarantine conditions where issues of air flow and environmental contamination are known and controllable – however, given the large number of passengers, this would have been impractical.”

Schaffner at Vanderbilt said the Japanese authorities likely hoped the ship would be an effective place to quarantine those infected.

“I don’t think anyone anticipated there would be such widespread, rapid transmission of the virus aboard the ship,” said Schaffner. “It is clear now that people underestimated the ability of this virus to spread.” ■