Of the roughly 150 passengers and crew members who boarded the MV Hondius, a luxury cruise ship, originally destined for Antarctica, three are now dead from a rare infectious disease: a strain of hantavirus known as Andes virus. Three more were evacuated to hospitals on Wednesday after developing symptoms like fever and shortness of breath. Some other passengers who may have been infected disembarked before the outbreak was recognized, raising fears among health officials that the virus, which can have a mortality rate up to 30 to 40 percent, could spread more widely.

The current outbreak is notable to me as an epidemiologist who studies infectious diseases because hantaviruses, a family of viruses spread by wild rodents that can cause lung and kidney diseases, don’t cause many outbreaks, despite being found all over the world. In fact, person-to-person transmission is exceedingly rare. If it is confirmed in this case, as I suspect it will be, it will make this event remarkable.

Despite the unsettling parallels to the early days of Covid-19, when cruise ships were among the first sites to be hit, the risks from this virus are quite different. Most hantaviruses infect only people who come into direct contact with infected rodent droppings, urine or saliva. Even the Andes virus, a strain that has been confirmed to pass between people, is a relatively poor spreader.

In previous outbreaks of hantavirus, person-to-person transmission has required sustained exposure, such as prolonged, close contact with a sick person, which is why the Andes virus strain, despite causing severe illness, is unlikely to cause an epidemic.

The close quarters of the people on the cruise may have given the hantavirus an unusual opportunity to spread through close contact in shared spaces. Based on what we currently know, only people who were aboard the cruise ship and their close contacts are at risk.

Still, it’s unwise to be too trusting when it comes to outbreaks. Viruses are wily. In the days and weeks ahead, there are a few developments I will be watching for that would change my level of concern.