This aerial picture shows a general view of the cruise ship MV Hondius stationary off the port of Praia, the capital of Cape Verde, on May 3, 2026. (Photo by AFP via Getty Images)AFP via Getty ImagesThree passengers are dead and three more are seriously ill aboard the MV Hondius, a polar expedition ship now anchored off Cape Verde. The World Health Organization confirmed Sunday that one case of hantavirus has been laboratory-confirmed, with five additional suspected infections. Hantavirus on a cruise ship is something that, as far as anyone can tell, has never happened before. “I don’t know of any other cases reported on a cruise ship,” Emily Abdoler, an infectious disease physician at the University of Michigan, told the New York Times. The ship left Ushuaia, Argentina, on March 20 carrying about 150 passengers, bound for Cape Verde by way of Antarctica, the Falkland Islands, South Georgia and a string of remote South Atlantic islands.The question epidemiologists are asking right now is not just how bad this outbreak will get. It is how six people on a cruise ship contracted a disease that humans typically catch by inhaling dust contaminated with rodent urine and droppings.Liver tissue revealed in the micrograph image from a hantavirus pulmonary syndrome patient, 1994. Image courtesy Centers for Disease Control (CDC). (Photo by Smith Collection/Gado/Getty Images)Getty ImagesMORE FOR YOUWhat Hantavirus IsHantaviruses are a family of viruses carried by rodents. They come in two broad categories. Old World hantaviruses, found across Europe and Asia, cause hemorrhagic fever with renal syndrome, a kidney disease that kills between 1% and 15% of those infected depending on the strain. About 150,000 to 200,000 cases occur annually, mostly in China. New World hantaviruses, found in the Americas, cause hantavirus cardiopulmonary syndrome, which attacks the lungs and heart. This form is far rarer but far deadlier, with a case fatality rate around 35% to 40% among those who become ill. The Centers for Disease Control and Prevention has documented only about 890 cases in the United States since surveillance began in 1993.The Hondius departed from Patagonia, home territory for one particular New World hantavirus called Andes virus. And Andes virus has a property that no other hantavirus shares: it can spread from person to person.Cotton rat (Sigmodon hispidus), a hantavirus carrier, 2005. Image courtesy Centers for Disease Control / James Gathany. (Photo by Smith Collection/Gado/Getty Images)Getty ImagesThree ScenariosHow did six passengers get infected? There are three plausible explanations, and each one carries different implications.The first is exposure on shore. Passengers on expedition cruises go ashore frequently, often in remote and wild places. The Hondius itinerary included landings in Antarctica, the Falklands and South Georgia, among others. Rodent exposure could have occurred at any stop. But the geography narrows the list. Antarctica has no native rodents. South Georgia completed the world’s largest rodent eradication effort around 2015, clearing rats and mice from more than 100,000 hectares at a cost of roughly $11 million. The island now deploys biosecurity dogs to check every arriving vessel. The Falklands have introduced rodents, but hantavirus has not been documented there. That leaves Ushuaia itself, where passengers likely spent time before embarkation, as the most plausible site of environmental exposure. The incubation period for hantavirus, typically two to four weeks, is consistent with that timeline.The second possibility is that the ship itself has a rodent problem. Ships have carried rats for as long as there have been ships, and Seoul hantavirus, one of the Old World strains, has been linked to the global spread of infected rats via maritime shipping. Surveillance programs at ports in China and West Africa exist specifically because of this risk. If the Hondius harbored infected rodents, passengers could have inhaled aerosolized droppings in confined below-deck spaces.The third scenario is that it spread from person to person. This the most consequential. Andes virus is the only hantavirus for which person-to-person transmission has been documented. The most dramatic demonstration came in 2018 and 2019 in the small town of Epuyen in Chubut Province, Argentina. A single rodent-to-human infection sparked a chain of person-to-person transmission that produced 34 confirmed cases and 11 deaths. Three “super-spreaders” who attended crowded social gatherings drove the outbreak. Before public health authorities intervened with isolation and quarantine, the reproductive number was 2.12, meaning each infected person was, on average, infecting more than two others.A cruise ship is, in epidemiological terms, a crowded social gathering that lasts for weeks.What We Don’t Know YetThe confirmed case, a British national now in intensive care in Johannesburg, tested positive for hantavirus, but the specific strain has not been publicly identified. If it is Andes virus, this outbreak would represent something new: the virus jumping from the rural Patagonian communities where person-to-person transmission has previously been documented into an international travel context, with 150 passengers from multiple countries now dispersing across the globe.If it is Seoul virus, carried aboard by ship rats, the public health calculus changes. Seoul virus does not spread person to person and carries a lower fatality rate, but it would raise urgent questions about rodent control on expedition vessels operating in remote waters.The WHO is coordinating a multicountry response with all affected islands and nations. For now, the Hondius remains off Cape Verde with two symptomatic passengers still on board, awaiting medical evacuation.Six cases of hantavirus in a single cluster is noteworthy by any measure. The answer to how they got it will determine what comes next.