The World Health Organization warned on Thursday that more hantavirus cases could emerge after the disease killed three passengers aboard the cruise ship MV Hondius.

A fourth sick passenger was evacuated to Amsterdam the same day, as the vessel sailed toward the Spanish Canary Islands and health officials raced to trace the outbreak. The WHO said it expected the spread to remain limited if precautions were taken.

How many hantavirus cases have been confirmed on the cruise ship?

The WHO has reported five confirmed and three suspected hantavirus cases, including three deaths.

The cases are linked to passengers aboard the MV Hondius. Because the Andes virus strain has an incubation period of up to six weeks, WHO chief Tedros Adhanom Ghebreyesus said more cases could still be reported in the weeks ahead.

What is the Andes hantavirus and why is it dangerous?

Hantavirus is a rare respiratory disease usually spread through contact with infected rodents. It can cause respiratory and cardiac distress as well as haemorrhagic fevers. There are no vaccines and no known cure. The Andes strain detected on the Hondius is particularly concerning because, unlike most hantavirus variants, it can be transmitted between humans.

A passenger is believed to have contracted the virus before boarding the ship in Ushuaia, Argentina, on April 1, and then infected others as the vessel sailed across the Atlantic. Argentine health authorities said on Thursday they had not yet been able to confirm where the outbreak began, stating it was “not possible to confirm the origin of the infection” based on information provided so far.

Who has been infected and where are patients being treated?

People confirmed or suspected to have contracted the hantavirus are being treated or isolating in Britain, Germany, the Netherlands, Switzerland and South Africa. The Leiden University Medical Centre in the Netherlands announced an additional positive test after the WHO’s initial case count.

Three passengers were evacuated from the ship on Wednesday when it anchored off Cape Verde, and a fourth landed in Amsterdam on Thursday, according to Netherlands-based ship operator Oceanwide Expeditions.

The company said there were no symptomatic individuals remaining on board as the Hondius sailed toward Tenerife, where it was scheduled to arrive on Sunday.

What happened to the passengers who died?

A Dutch man who boarded in Ushuaia died aboard the ship on April 11. His body was removed at Saint Helena, a south Atlantic island, on April 24, where 29 other passengers also disembarked. His wife, who left the ship to accompany his body, later fell ill and died in South Africa on May 4, with hantavirus confirmed as the cause. A German passenger died on May 2, and her body remains on the ship.

The couple had visited Chile, Uruguay and Argentina before boarding. Chile’s health ministry said the pair were not infected in that country, as their travel there did not align with the virus’s incubation period.

The Dutch woman flew on a commercial flight from Saint Helena to Johannesburg while showing symptoms. Officials were working to trace the 82 passengers and six crew on that Airlink flight.

How are health authorities responding to the hantavirus outbreak?

The WHO said it had informed 12 countries that their nationals disembarked from the Hondius at Saint Helena. The Saint Helena government said more than 95 percent of its population had no close contact with the ship’s passengers or crew, and described the local risk of infection as “extremely low.” Oceanwide Expeditions said it was working to trace all passengers and crew who boarded or left the ship since March 20.

Argentina said it planned to test rodents in Ushuaia to help identify the source. The WHO’s emergency alert director, Abdi Rahman Mahamud, said he believed the outbreak would remain limited if “public health measures are implemented and solidarity shown across all countries.”

US President Donald Trump said Thursday he had been briefed on the situation. “It was the ship, and I think we’re going to make a full report about it tomorrow,” he told reporters. “It should be fine, we hope.”