An outbreak of hantavirus aboard a cruise ship in the Atlantic Ocean has heightened concerns about the rare but deadly virus. Public health officials say that the threat to the general public remains low based on what we know about the virus and how it spreads.

The outbreak involves the Andes strain of the virus.

There are many different strains of hantavirus, said Dr. Gaby Frank, director of the Johns Hopkins Special Pathogens Center in Baltimore. Laboratory testing has shown that the cruise outbreak involves the Andes strain, which is endemic to South America, including Argentina, where the ship departed on April 1. Dr. Maria Van Kerkhove, head of epidemic and pandemic preparedness and prevention at the W.H.O., said the initial cases of hantavirus were in a couple believed to be infected before they boarded in Argentina.

Most forms of hantavirus spread through contact with the feces, urine and saliva of infected rodents, including when humans breathe in particles of the virus. The Andes virus is the only strain known to spread from human to human, though such cases are rare.

Unlike the flu, Covid or even the common cold, it’s exceedingly difficult for the Andes strain to move between people, said Bryce Warner, a research scientist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan who has extensively researched hantavirus.

The flu and Covid primarily spread from the upper respiratory tract and replicate in high levels. Hantavirus sheds less easily, and also burrows deep into the lungs and infects blood vessels. “It’s not very easy for the virus to get out,” Dr. Warner said.

Those at highest risk of person-to-person transmission are people who have been in close quarters for prolonged periods of time with someone who has been infected, like health care workers or those who share a household. Two of the passengers who died on the ship were a married couple. The ship’s doctor showed symptoms of the virus and was evacuated from the ship. It isn’t clear yet how other people on board may have been infected.