The World Health Organization says there may have been rare human-to-human transmission of hantavirus on the Dutch cruise ship where three passengers have died.
The virus is usually spread from rodents, but the WHO said in this instance it could have spread among "really close contacts" aboard the MV Hondius vessel. It stressed the risk to the public was low.
Two crew members - one British and one Dutch - are due to be medically evacuated by aircraft to the Netherlands after displaying "acute respiratory symptoms", the ship's operator Oceanwide Expeditions said. A person linked to a German national who died is also due to be evacuated.
MV Hondius set sail from Argentina on its voyage across the Atlantic Ocean around a month ago. It is currently anchored near Cape Verde, off Africa's west coast.
Medical teams from Cape Verde, supported by the WHO, have boarded the ship to help with the suspected cases, spokesperson Tarik Yasarevic told the BBC. Testing is taking place for other passengers and crew members who are displaying symptoms.
Images taken from on board the cruise ship show workers in hazmat suits in a smaller vessel alongside.
Some 149 people from 23 countries remain aboard under "strict precautionary measures", Oceanwide said. Aside from the crew member due to be evacuated, there are 22 other British nationals aboard.
"We do believe that there may be some human-to-human transmission that is happening among the really close contacts," WHO official Dr Maria Van Kerkhove said.
She added that the WHO suspected the first person to fall ill could have contracted the virus before boarding the ship.
Seven cases of hantavirus - two confirmed and five suspected - have so far been identified, according to the latest WHO update.
The two confirmed cases are a Dutch woman, who is among those who died, and a 69-year-old UK national who was evacuated to South Africa for medical treatment.
The woman's husband also died but he is not a confirmed case, nor is the German national who passed away on 2 May.
In a statement, the Dutch couple's family said: "The beautiful journey they experienced together was abruptly and permanently cut short."
"We are still unable to comprehend that we have lost them. We wish to bring them home and commemorate them in peace and privacy," it added.
Investigators are working under the assumption that the Andes strain of the virus, which spreads in South America where the cruise began, has been found in the two confirmed cases.
The organisation was told there were no rats on board, Van Kerkhove said, adding that disinfection was taking place on the ship and those with symptoms or caring for patients were wearing full personal protective equipment.
"Our working hypothesis is that there's probably a couple of different types of transmission that might be happening," Van Kerkhove told BBC Breakfast on Tuesday.
She noted the cruise had visited many different islands, some of which have rodents, which typically spread the virus through their faeces, saliva or urine.
The WHO said Spain has granted permission for the vessel to dock in the Canary Islands, where a risk assessment and further medical monitoring could take place.
But Spain's health ministry has played down speculation that it will take in the ship.
It said in a statement: "Depending on epidemiological data gathered from the boat as it travels past Cape Verde, the most appropriate next stop for it will be decided.
"Until then, the health ministry will not take a decision, as we have explained to the WHO."
A spokesman for Spain's health ministry told the BBC it had not yet received a request for the boat to stop at the Canaries.
However, Spanish authorities are prepared to take charge of the situation should that change, the spokesperson added. That would include providing medical attention, analysis and disinfection.
They would not say whether passengers would be allowed to disembark.
Though they cannot yet leave the ship, one passenger told the BBC on Monday that the mood on the vessel was "pretty good".
"Hopefully the other patients on board will be tested soon and then we'll know what's going on," the passenger, who asked to remain anonymous, added.
Another passenger, travel vlogger Jake Rosmarin, said in a social media post: "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."
With additional reporting by Pumza Fihlani
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