Hantavirus, Covid, norovirus, legionnaires’: Why are cruise ships so prone to disease outbreaks?

Vikram Niranjan For The Conversation

Limerick (Ireland): 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 cruise ship.

This helps explain why ships such as Celebrity Mercury, Explorer of the Seas and Carnival Triumph have become familiar names in outbreak reports. These were not unusual in some special way; they were simply settings where shared dining, close contact and frequent movement through common areas allowed infection to spread fast.

Food service plays a big part in this risk. Buffet-style dining, shared utensils and many people touching the same surfaces can make it easier for stomach bugs to spread. If someone is infected but does not yet feel sick, they may still contaminate food or surfaces before they realise they are unwell.

The ship’s design adds to the problem. People spend time together in dining rooms, bars, lifts, corridors, theatres and spa areas. Crew members also live and work in the same environment, often in shared accommodation, so illness can move through the ship from passenger to passenger or between passengers and crew.

Ventilation also plays a crucial role. Cruise ships are not closed boxes, but they do rely heavily on indoor spaces where people spend long periods together.

Studies into cruise ship air quality have shown that illness can spread more easily in crowded, enclosed spaces, like cabins, restaurants and entertainment venues, if the ventilation system is not up to scratch. Things like adequate fresh air circulation, specialist filters and air-purifying technology all play a role in keeping passengers safe.

Legionnaires’ disease (a serious lung disease caused by bacteria) shows a different kind of risk. It is not usually spread directly from one person to another. Instead, people can get infected by breathing in tiny droplets from contaminated water systems, hot tubs or showers.

A well-known outbreak among cruise passengers was linked to a whirlpool spa, and recent reports from the US Centres for Disease Control and Prevention have described other cruise-associated legionnaires’ disease outbreaks linked to ship water systems.

Age also matters. Cruise holidays are especially popular with older adults, and many passengers have long-term health conditions that make infections more serious. A stomach bug on a cruise can lead to dehydration, and a respiratory infection can lead to pneumonia or hospital care.

Cruise ships do have medical facilities, but they are limited compared with land-based hospitals. They are built to give first aid, basic treatment and short-term care, not to manage a fast-moving outbreak on a large scale. That is why cruise health depends so much on early reporting, quick isolation and strong cleaning practices.

Other infections, such as respiratory viruses, including influenza, can spread in the same crowded indoor settings, and stomach bugs can spread through food, hands and shared surfaces.

COVID and flu exploit enclosed air and crowds. Norovirus loves buffets and surfaces. Legionnaires’ targets water systems, which ships can’t easily sterilise. Hantavirus (a severe respiratory illness spread by rodents) outbreaks on ships are rare. However, as recent news of the deaths on the MV Hondius attests, germs in close quarters find it much easier to spread.

How to limit your risk

As an epidemiologist, I have seen many outbreaks in hospitals, schools and even on flights. For travellers, the best protection starts before boarding. It is sensible to check whether the cruise line has clear illness reporting, cleaning and isolation policies. Make sure your routine vaccines are up to date.

And for older adults, pregnant women and anyone with health problems, consult your GP before travelling. Also, ensure your travel insurance covers illness-related disruptions.

Once on board, washing your hands with soap and water is the most useful step for preventing stomach bugs like norovirus. Hand sanitiser can help, but it does not replace soap and water. If you start to feel unwell, the safest move is to avoid buffets and crowded shared spaces and report symptoms early rather than trying to carry on as normal.

Cruise lines have improved their hygiene and outbreak response systems over time, and many voyages pass without incident. But the basic structure of cruise travel still creates the same challenge: many people sharing the same meals, the same air, the same water systems and the same common spaces.

That is why outbreaks keep returning, and why cruise ships remain a useful reminder that public health is shaped as much by design as by germs.

The author is Assistant Professor in Public Health, School of Medicine, Health Research Institute, University of Limerick.