A suspected hantavirus outbreak on a cruise ship in the Atlantic Ocean has raised questions about how the rare, often fatal illness could have spread among passengers and crew, and whether it may continue to infect some 150 people now stranded aboard the vessel after three already died.

While typically transmitted to humans through contact with contaminated rodent waste, health officials say it is possible for transmission to take place between people, albeit on a very limited basis. That may partially explain what occurred in this instance, said Maria Van Kerkhove, the World Health Organization's director of epidemic and pandemic preparedness and prevention.

"We do believe that there may be some human-to-human transmission that's happening among the really close contacts, the husband and wife, people who've shared cabins," Van Kerkhove told reporters at a news conference Tuesday, referencing a couple who died from suspected hantavirus cases after spending time on the MV Hondius, a Dutch ship that became the focal point of the apparent outbreak while charting a weekslong polar expedition.

An investigation, quarantine protocols and contact tracing efforts were underway in hopes of pinpointing the origins of the suspected outbreak and staving off further spread, but exactly how it may have emerged was not yet clear, WHO said in a statement. At the news conference, Van Kerkhove suggested the husband and wife may have contracted the virus while exploring wildlife in Argentina, before they joined the cruise. She said the U.N. health agency is generally operating under the assumption that the virus came from outside of the ship.

"The cruise did stop at many different islands up the coast of Africa, and, again, seeing a lot of different wildlife on those islands. There are birds. Some islands have a lot of rodents. Others don't," said Van Kerkhove. "So, there could be some source of infection on the islands as well for some of the other suspected cases."

One known hantavirus strain spreads between people

Hantaviruses are a family of diseases that can cause severe respiratory symptoms, and even death, in people worldwide, according to the U.S. Centers for Disease Control and Prevention. People usually contract the virus after coming in contact with airborne particles from rodent droppings, urine or saliva. But, as WHO said in a statement about the latest outbreak, there have been reported instances of human-to-human transmission of Andes virus, a specific strain of hantavirus that is responsible for most cases of the illness in South America. 

"This cruise ship took off from Argentina, which is why we're wondering, is it the usual kind [of hantavirus], or is it a kind that can be transmitted person-to-person?" said CBS News medical correspondent Dr. Céline Gounder on Tuesday. 

The largest outbreak of the Andes strain was in Argentina in 2018, leading to 34 cases and 11 deaths, Gounder said. But, human-to-human transmission requires "prolonged close contact," she said, adding, "this is not a pandemic kind of virus."

She also emphasized that hantavirus remains rare, with fewer than 900 cases over 30 years in the U.S. 

The cruise operator, Oceanwide Expeditions, said a 70-year-old Dutch man died on board the ship on April 11. The man's 69-year-old wife died about two weeks later in South Africa, after disembarking the ship, officials said. According to South Africa's Health Ministry, the wife's blood tested positive for hantavirus. A British passenger evacuated from the Hondius to South Africa on April 27 also tested positive, authorities said, and health officials have described him as critically ill.

There are at least four more suspected cases, said Van Kerkhove, including another fatal infection in a German passenger whose body was still on board the ship. At least three others were sick, according to Oceanwide Expeditions and Ann Lindstrand, a WHO representative for African island nation of Cape Verde. On Monday, Cape Verde sent medical teams to assist the Hondius crew but prevented the ship from docking at its ports because of health concerns.

Lindstrand told CBS News that officials strongly believe the three sick people had contracted hantavirus and said all would be evacuated from the Hondius by helicopter on Tuesday. 

She agreed the virus was likely contracted off the ship and said no rodents were found aboard the Hondius. Traps on the ship would show signs of them if they had been there, she said. 

Lindstrand sought to assuage fears of cruise ships, which are seldom linked to hantavirus despite being frequent sites of other kinds of disease outbreaks, like norovirus. Asked to what extent hantavirus is a pandemic-level threat, she said: "It's not."

"We hear you"

The ship was expected to make its way toward Spain's Canary Islands after the sick passengers are evacuated. The Spanish Ministry of Health told CBS News in a statement that its epidemiologists would conduct a review of the ship Tuesday afternoon "to assess the condition of the people on board, determine whether there are more individuals with symptoms, and identify any high-risk or low-risk contacts," which "will help inform decisions regarding repatriation processes and the ship's route."

The passengers and crew could potentially be kept on board for up to eight weeks, said Lindstrand, since the virus' incubation period can be that long. However, she acknowledged the psychological toll such "distressing" circumstances could take. 

One passenger, travel blogger Jake Rosmarin, touched on that in an emotional social media video shared Monday, saying, "There's a lot of uncertainty, and that's the hardest part. All we want right now is to feel safe, to have clarity and to get home."

Officials recognized that the situation is frightening for those on the ship.

"We have heard from quite a few people on the boat," said Van Kerkhove at Tuesday's news conference. "We just want you to know we are working with the ship's operators. We are working with the countries where you are from. We hear you. We know that you are scared."