It was a voyage that promised such stuff as dreams are made of, yet within weeks the Atlantic expedition of the MV Hondius had become a nightmare, with three passengers dead from hantavirus and more showing symptoms.

Meanwhile, an outbreak of norovirus is under investigation on another cruise ship, while flu, E coli and varicella – the virus that causes chickenpox – have also caused problems in such settings. Perhaps most memorably, in 2020 the Diamond Princess became a breeding ground for Covid, with passengers and crew quarantined for two weeks off the coast of Japan and more than 700 of the 3,711 people onboard eventually testing positive.

In some ways, it is hardly a surprise that cruise ships can become hotbeds of infection: many are floating behemoths where those onboard interact frequently and at close quarters. Researchers also note that many of those onboard will have come from different countries, have travelled to different locations and have different degrees of immunity to diseases.

What’s more, the ship moves. “Which means you have people potentially in contact with pathogens that they don’t encounter on the day to day,” says Dr Charlotte Hammer, an infectious diseases epidemiologist at the University of Cambridge.

Throw in that a substantial proportion of passengers tend to be older adults, who may be more susceptible to certain diseases, and the scene appears set for potential disaster.

David Heymann, a professor of infectious disease epidemiology at the London School of Hygiene & Tropical Medicine, says: “Whether or not [outbreaks] begin on a ship depends on who comes onboard, if any people are infected when they come onboard, and with which pathogen they might be infected.”

As Heymann points out, infections can be transmitted in different ways. One is through respiratory transmission between people, either via aerosols – tiny particles that linger in the air and can be inhaled – or via droplets that can either fall on people directly or contaminate surfaces. Covid and flu are among the diseases spread this way.

While many ships have made efforts to improve their ventilation to boost air quality, Hammer notes there are limits to what can be done. “You’re not going to have high ceilings on a boat. You are not going to have the airflow of two open windows, just because most cabins do not have windows,” she says. “So there is, in terms of the sheer engineering of a ship, only so much you can do.”

Another route by which diseases can be spread is through contaminated food, as is often the case for E coli and norovirus outbreaks. Hammer says a key issue on cruise ships is that while their kitchens have high levels of hygiene, they also provide a single point of failure. “Again, that is sort of the core engineering. You can’t have X number of backup kitchens on a ship because you only have so much space.”

Dr Vikram Niranjan, an assistant professor in public health at the University of Limerick, says buffets are a potential point of spread, not least as everyone shares the same serving utensils. And there are other surfaces that are regularly touched. “Ships aren’t dirty, they are just efficient mixing chambers,” he says.

There is also the possibility of outbreaks that spread via water systems. Cruise ships have previously had outbreaks of legionnaires’ disease, a lung infection caused when people inhale droplets of water containing legionella bacteria. “That’s very difficult to deal with,” Heymann says. “First of all, you have to show that the organism is in the water, and the ship may not have the means to do that.”

Once an outbreak has begun, cruise ships face challenges, not least in diagnosis. Hantavirus, for example, is rare, and only one strain is known to pass between people. As a result, the medical team onboard – which sometimes includes just one doctor – may not immediately realise what they are facing. “You would begin to suspect if a lot of people had it. But if you saw just one hantavirus infection, it looks like any other viral infection,” Heymann says.

As Hammer notes, ships do not have full laboratories or a whole suite of hospital equipment. “You have limitations in terms of testing, particularly for something that’s a bit rarer,” she says. “You have limitations in terms of medical space, medical professionals.”

Niranjan says while cruise ships often have a space for isolation and ventilators, they are not prepared for mass outbreaks. He says one option could be to have collapsible isolation cabins that could be used in an emergency.

Heymann says another change that could be made is for the larger cruise ships to ensure their doctors are better trained for such scenarios. “Hopefully, these doctors will begin to get more skills in epidemiology and in dealing with outbreaks so that when an outbreak does occur on a ship, they can minimise the transmission.”

Passengers can take precautions. The US Centers for Disease Control and Prevention issued guidance emphasising not boarding if unwell, regular handwashing when onboard, getting vaccinated against routine diseases, having destination-specific vaccines or medications, and notifying the ship’s medical centre if you are sick. It also mentions the importance of having travel insurance. Niranjan adds it could be worth packing face masks.

Hammer says many of the factors that put cruise ships at risk of outbreaks are not necessarily easy to change. “Or rather, if you change them, the cruise ship is no longer a cruise ship,” she says. “You can make it not move any more – but that sort of defeats the point.”