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 — which is typically spread by exposure to rodent urine, feces or saliva and can be deadly — may have spread between passengers on a cruise ship that's anchored off the coast of Cape Verde.

"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," said Maria Van Kerkhove, the director of epidemic and pandemic management at the World Health Organization, speaking at a press conference on Tuesday.

"It's very, very surprising, and obviously a very rare occurrence," says 

, a virologist at the Johns Hopkins Bloomberg School of Public Health. However, she adds, the evidence presented by the WHO is "compelling," although the risk to the general public remains very low.

As of Wednesday, there are three confirmed cases and five suspected hantavirus cases among the 147 passengers and crew members on the boat.

Three of the patients have died and one patient is in intensive care in South Africa, although Van Kerkhove said this patient is "improving." She added that two people on board the ship are being prepared for medical evacuation to the Netherlands, where they will receive treatment. Another suspected patient had a fever but is currently asymptomatic.

The final confirmed case is a previous passenger on the ship who presented himself to a hospital in Zurich after receiving an email from the ship's operator about the health situation. He's now receiving care.

Hantavirus, a rare but serious virus, can cause hantavirus pulmonary syndrome, in which initial flu-like symptoms — including fatigue, fever and muscle aches — give way to severe respiratory symptoms as the lungs fill with fluid. According to the 

Centers for Disease Control and Prevention

, more than a third of patients who get respiratory symptoms "may die" from the condition.

Humans are typically infected when inhaling hantaviruses that have become airborne from rodent excretions. However, in a small number of case studies it appears the virus can be transmitted between people.

"There are like 20 to 30 different species of hantavirus worldwide that can cause human disease, and there is only one [of those] species — the Andes Virus, which is found in Argentina and Chile — that has been implicated in human-to-human transmission," explains Dr. Emily Abdoler, a clinical associate professor of medicine at the University of Michigan. "One of the first clues that emerged is that this ship disembarked from Argentina."

 that samples from the outbreak were analyzed by the National Institute for Communicable Diseases in South Africa and the Geneva University Hospitals in Switzerland and were confirmed by both as Andes hantavirus.

The next clue that this could be human-to-human transmission is based on the timing between when a person is exposed to the virus and becomes sick — which can be anywhere between one and eight weeks. The first person to become sick — a man who traveled in Argentina before joining — got ill within the first week of the cruise and died shortly after. 

The other patients became sick a couple weeks later. Van Kerkhove said, "Our assumption is [the first patients] were infected off the boat and then joined the cruise.

Van Kerkhove said it is possible that people on the boat were exposed to rodents elsewhere — and did not contract the virus from the original patient.

She said the cruise ship is an expedition boat, and many people onboard were stopping at islands along the coast of Africa for activities like bird watching. "On those islands, there are birds. Some islands have a lot of rodents. Others don't. So there could be some source of infection on the islands as well for some of the other suspect cases," she says.

However, Abdoler says, having the potential of human-to-human spread means the public health response is different. 

"If it were just rodents on the ship, then taking people off the ship and not exposing other people to the rodents on the ship should be enough to stop the spread," she says. But with the Andes virus present and potential human-to-human transmission, "then taking folks off the ship doesn't stop the spread."

She says that's why the public health response includes isolation and quarantining folks who have had contact with passengers, even beyond the ship."

Debbink says it's not entirely known how the Andes Virus transmits between people. "If I were on the ship and I had a mask, I would probably be masking," she says, adding that people will have to be monitored for many weeks because it can take a while for the virus to make someone sick.

However, Debbink says, if this is indeed human-to-human transmission, the virus does not seem to be highly transmissible because then "you would have a lot more cases on the cruise ship, just from people being around each other in pretty close proximity."

WHO's Van Kerkhove says that anyone interacting with the patients is wearing full personal protective equipment and that the medical personnel who have boarded the boat have brought additional protective equipment with them.

The current plan, according to Van Kerkhove, is for the cruise ship to continue on to the Canary Islands, where Spanish authorities have said they will help do a full epidemiologic investigation, the ship will be disinfected and the other passengers on board will be assessed.

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Gabrielle Emanuel

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While most hantaviruses spread through contact with rodent feces, urine or saliva, officials confirm that the type on the cruise ship is Andes hantavirus, which is known to spread between people.

Hantavirus on cruise ship confirmed as rare type that can spread human-to-human

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