The World Health Organisation ordered the evacuation of two passengers on Tuesday from the Dutch cruise ship MV Hondius after a Hantavirus cruise ship outbreak killed three people.
Dr Maria Van Kerkhove, WHO Director for Epidemic and Pandemic Preparedness and Prevention, said officials could not rule out human-to-human transmission because some cases involved very close contact.
WHO said its medics expected to board the vessel to help evacuate people. The deaths included a Dutch couple and a German national, according to the reported case details. A British national remained in intensive care in South Africa.
About 150 people, including 17 US nationals, remained onboard the MV Hondius as it sailed off the coast of West Africa.
The outbreak was first reported on Sunday, May 3, on the Netherlands-based vessel during a voyage between Argentina and Cape Verde.
WHO officials had earlier said there was no need for panic and that the public risk was low, only for authorities to issue the evacuation order on Tuesday, May 5.
People generally associate hantaviruses with rodent exposure, but the WHO said it was treating close-contact transmission as a precaution in the current outbreak.