The Dutch cruise ship MV Hondius at the centre of the deadly outbreak departed Cape Verde with nearly 150 people on board on Wednesday and headed to Spain's Canary Islands.
Associated Press footage showed health workers in protective gear taking away three passengers, including the ship's British doctor, who Spain's health ministry said had been in "serious condition" but has improved.
An air ambulance later departed.
On Wednesday evening, a medical evacuation flight arrived at Amsterdam's airport.
Three people have died, and one body remained on the ship, the WHO said.
Of the eight cases recorded, five were confirmed by laboratory testing.
Hantavirus usually spreads by inhaling contaminated rodent droppings and can spread human-to-human although that is rare, according to the WHO, whose top epidemic expert said the risk to the public is low.
Health officials in Europe and Africa are trying to identify people who may have had contact with people who earlier left the ship, which departed on April 1 from South America for stops in Antarctica and several remote Atlantic islands.
Two Argentine officials investigating the origins of the outbreak said the government's leading hypothesis is that a Dutch couple contracted the virus while bird-watching in the city of Ushuaia before boarding.
They said the couple visited a landfill during the tour and may have been exposed to rodents.
The officials spoke on condition of anonymity because they were not authorised to brief the media, with the investigation ongoing.
Authorities previously said Ushuaia and surrounding Tierra del Fuego province had never recorded a hantavirus case.
The Dutch foreign ministry said the three people who left the vessel were a 41-year-old Dutch citizen, a 56-year-old UK citizen and a 65-year-old German who would be transferred to specialised hospitals in Europe.
The WHO said on Wednesday that testing in Senegal confirmed that two of those transferred were infected with hantavirus.
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