The deceased include an elderly married couple from the Netherlands and a British citizen. At least three other people, including two crew members, have fallen ill.
So far, at least one case of the virus has been confirmed, the World Health Organization said. The virus is most commonly spread by rodents, but human-to-human transmission can also occur, the organization said.
In total, around 150 tourists and 70 crew members were on board the ship when the outbreak is believed to have occurred.
The cruise ship is currently off the coast of Cape Verde, waiting to be allowed to dock safely.
