This article is republished from The Conversation.

Cruise ships are convenient floating hotels by which to see far-flung parts of the world – but as an epidemiologist, I know they are also everything an infectious pathogen could want: thousands of strangers packed into enclosed spaces for days or weeks, sharing dining rooms and high-touch surfaces such as elevator buttons and handrails, breathing recirculated air.

Each new port of call where passengers can explore for a few days is an opportunity for germs to embark – and once they do, they encounter a highly efficient setting for hopping from host to host.

The MV Hondius confirmed this well-known fact in April 2026, when an outbreak of Andes hantavirus began aboard the Dutch-flagged expedition vessel carrying 147 passengers and crew from 23 countries.

The Andes virus is one of several species of hantaviruses. It is the only one known to spread from person to person, though it doesn't do so very efficiently. It is far less contagious than COVID-19 or the measles.

As of May 14, a total of 11 cases, including three deaths, have been reported in the Hondius outbreak.

Outbreaks at sea are one of the oldest problems in public health. From medieval plague quarantines to modern times, they have repeatedly tested the ability to control infectious disease – and have played a key role in shaping the international public health framework in place today.

That interconnected public health system, however, depends on the cooperation of countries around the globe.

From harbor quarantine to global disease control

The word "quarantine" was first documented in the English language in 1663, in the Oxford English Dictionary, which defined it as a period of 40 days during which people who might spread a contagious disease are kept isolated from the rest of the community.

The first official quarantine, though, came earlier, in 1377, when the Republic of Ragusa – modern-day Dubrovnik, Croatia – ordered ships from plague-affected ports to anchor offshore for 30 days before anyone could disembark. A quarter-century later, Venice extended this period to 40 days – hence the "quarantine" term, which stuck. In 1423, Venice officially opened the world's first permanent quarantine island, the Lazzaretto Vecchio, specifically to manage the problem of the plague arriving by sea.

The system worked during the medieval era because a single authority usually controlled most harbors. Ships waited because they recognized states' authority to detain them.

For centuries, maritime quarantine operated on this principle. Harbor officials wielded broad public health powers over incoming vessels. In the 19th century this practice continued in the United States. Cholera ships – a nickname for trans-Atlantic vessels carrying migrants and troops that were breeding grounds for cholera and other diseases – arrived from Europe and the Mediterranean and sat offshore in New York for weeks. At quarantine stations on Ellis Island and ports across the Atlantic seaboard, ships were inspected, passengers isolated and captains overruled by public health officers who had the legal authority to isolate passengers for extended periods.

The system was crude and often brutal. Ships of the medieval period were floating sickrooms with poor conditions: putrid water in the casks, bread full of worms, and passengers packed into pitch-sealed berths with lice in the bedding and the bilge stinking under them. Many people died on board. But the system rested on a foundation of recognized, enforceable authority over the vessel and everyone on it for the purpose of protecting the city from disease.

International cooperation

As maritime trade and travel became increasingly globalized, however, no single port or government could manage outbreaks alone. Also, advances in vaccines, antibiotics and sanitation led many countries to downsize the maritime quarantine systems that had once defined disease control at sea.

This forced quarantine systems to evolve from local harbor control into international frameworks for coordination. The World Health Organization was established in 1948, and the International Health Regulations were created in 1969 to manage disease across borders.

Countries agreed to share information, notify one another of outbreaks and coordinate responses at ports and borders. The responsibility no longer fell on a sole harbormaster, but the system was designed to perform a similar coordinating function across an increasingly interconnected world.

Even within that system, however, cruise ships remain unusually vulnerable outbreak environments. A highly visible example was a COVID-19 outbreak that occurred on the Diamond Princess in 2020. The cruise ship, which was anchored off the coast of Yokohama, Japan, produced weeks of confusion between Japanese authorities, the British cruise operator and a dozen foreign governments as they struggled to coordinate responsibility for the 3,700 passengers and containment measures.

Some analyses later suggested the shipboard quarantine may have amplified transmission. At the time, most observers treated it as a crisis specific to the early chaos of the pandemic.

But the Hondius outbreak suggests the problem runs deeper.

Ships cross borders – so too do pathogens

Cruise ships combine dense social mixing, international mobility and fragmented legal authority in ways that continue to challenge modern disease-control systems – even decades after the creation of international public health frameworks designed to coordinate them, and even for diseases like Andes hantavirus that are extremely unlikely to cause a pandemic.

As the cruise industry has grown, it has expanded into more remote and epidemiologically unpredictable environments – expedition voyages to Antarctica, the Amazon, Alaska. Alongside the industry's ambitions, disease risk has also increased. These trips routinely bring large groups of passengers into contact with wildlife, pathogens and ecosystems they may have little prior exposure to and then seal travelers together for weeks.

Nevertheless, the United States chose in January 2026 to withdraw from the World Health Organization, the primary institution administering the framework designed to coordinate responses when disease crosses the borders that cruise ships cross as a matter of routine.

The Trump administration framed the exiting of international organizations as a means of protecting U.S. sovereignty. In practice, it meant that when the Hondius needed a response, the U.S. participated from outside the systems it had spent decades helping to build.

A crack in the system

In the outbreak on the Hondius, the international system still functioned.

The WHO still issued risk assessments and guidance. The European Centre for Disease Prevention and Control still coordinated the response across Europe. And in the U.S., the Centers for Disease Control and Prevention belatedly issued a health alert to physicians.

What changed is that the U.S. moved from being a central participant in the international public health system to operating more from its edges.

Who can say whether the next big outbreak will come from a disease spread on a cruise ship – or whether the pathogen involved will be one that spreads more efficiently between people than the Andes strain of the hantavirus does.

Whatever its source, outbreak response depends on cooperation between major governments, rapid information sharing and coordinated logistics. When a country as globally connected as the U.S. steps back from those systems, managing international health emergencies becomes slower, more fragmented and more dependent on ad hoc negotiations. Ultimately, this may make the world less safe.