Three 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.

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Hantaviruses 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........

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