Health officials believe a rare strain of hantavirus that spreads from person-to-person and has a 40% mortality rate is responsible for the outbreak on the MV Hondius cruise ship that has killed three passengers, and sickened seven others.

Though hantavirus is typically only spread through rodent droppings, this particular strain is an alarming exception – and it’s one that “can’t be ruled out,” the World Health Organization conceded Tuesday.

“We do believe that there may be some human-to-human transmission that is happening among the really close contacts,” said Maria Van Kerkhove, the World Health Organization’s epidemic and pandemic preparedness and prevention director.

The exceptional strain, “the Andes virus,” is only found in the mountains of Argentina and Chile. The MV Hondius started its weeks-long voyage in Argentina on March 20, but not in an area where that strain is usually found.

The Andes hantavirus strain carries a mortality rate of nearly 40%, meaning a worst-case scenario could spiral quickly in a confined environment such as the 80-cabin, 353-foot ship with roughly 150 people on board.

The terrifying strain seemed even more likely after the WHO said no rodents were found on board the vessel, which remained anchored off the West African island nation of Cape Verde Tuesday.

Medical teams wearing full personal protective gear have reportedly boarded the ship to help with suspected cases in two crew members – one British and one Dutch – and test other passengers and crew members who are showing symptoms of hantavirus.

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Here’s the latest on the deadly hantavirus outbreak:

• Here’s what’s next for doomed hantavirus-infected cruise ship
• The two possible causes for the cruise ship hantavirus outbreak — one is disgusting, the other terrifying
• MV Hondius hantavirus cases rise to 7 as cruise passengers remain trapped off Cape Verde: WHO
• Inside the deadly hantavirus-infected cruise ship, MV Hondius, where 3 passengers have died
• American cruise passenger sobs about being stuck on ship where 3 have died in hantavirus outbreak

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The WHO believes the first passenger to fall ill may have contracted the virus before boarding the ship, Van Kerkhove said.

“Our working hypothesis is that there’s probably a couple of different types of transmission that might be happening,” she told BBC Breakfast Tuesday, and noted the cruise had visited many different islands, where passengers may have come in contact with rodents, which can spread the virus through their faeces, saliva or urine.

The WHO said Spain granted the boat permission to dock in the Canary Islands, but Spain’s health ministry reportedly refuted the idea that it would definitely take the ship.

“Depending on epidemiological data gathered from the boat as it travels past Cape Verde, the most appropriate next stop for it will be decided,” the ministry said in a statement.

“Until then, the health ministry will not take a decision, as we have explained to the WHO.”