Tests on patients in South Africa and Switzerland showed it was a hantavirus found in South America, called the Andes virus, officials said.

Hantavirus usually spreads by inhaling contaminated rodent droppings and can spread person-to-person, though that is rare, according to the WHO, whose top epidemic expert said the risk to the public is low.

As the number of confirmed infections increased to five, health authorities in three continents were investigating the source and tracing dozens of people who might have come in contact with passengers who left the ship earlier.

What happened on the ship

Oceanwide Expeditions, the Dutch company that operates the MV Hondius, offers “expedition cruises” that involve trips to the Antarctic and several islands in the South Atlantic to see some of the remotest places on Earth.

The cruises can last a month or more and cost between $6,000 and $25,000, depending on the cabin.

The Hondius set off from southern Argentina on April 1.

On April 6, the 70-year-old Dutch man fell ill with fever, headache and diarrhea, WHO said.

He died on board on April 11, after developing respiratory distress. The ship was between the British island territories of South Georgia and St. Helena in the middle of the South Atlantic, according to data from the ship tracking website MarineTraffic. The cause of death could not be determined, according to Oceanwide Expeditions.

The ship sailed on for nearly two weeks, stopping near the island of Tristan da Cunha before reaching St. Helena, where the Dutch man’s body was removed on April 24. His 69-year-old wife disembarked.

The woman, who already had symptoms, became sicker during an April 25 flight to South Africa and collapsed at an airport there. She died at a hospital on April 26, WHO said.

The patient in Switzerland also disembarked in St. Helena, according to Swiss authorities, though his movements after that are not clear.

Another passenger, a British man, became sick on the ship after it left St. Helena and sailed to tiny Ascension Island, some 1,300 kilometers (800 miles) north. He had a high fever, shortness of breath and signs of pneumonia, according to WHO, and was evacuated from Ascension Island to South Africa on April 27. He is in intensive care in South Africa.

The third fatality, a German woman, died on the ship on Saturday, again after it had set sail for a new destination — this time Cape Verde. She died four days after falling ill and also had signs of pneumonia, WHO said, which can be caused by hantavirus. Her body is still on the ship.

When authorities first knew hantavirus was on the ship

Health officials in South Africa tested the British man in intensive care for hantavirus after tests for other ailments were negative. They received a positive result for hantavirus on Saturday, 21 days after the first passenger died.

On Sunday, WHO announced it was investigating a suspected hantavirus outbreak on the ship, which had by that time reached Cape Verde waters.

The British man’s positive test prompted South African health authorities to test the Dutch woman’s body. That test came back positive on Monday.

wiss authorities announced the positive test on the man there on Wednesday.

Contact tracing was underway.

The plan for the people still on board

After waiting off Cape Verde for three days, the ship headed to the Canary Islands, where Spain said it would accept it. People on board are from Britain, the United States, Spain, Netherlands, Germany and more than a dozen other countries.

Passengers and crew have been isolated in cabins with “physical distancing,” WHO said, in a lockdown reminiscent of the COVID-19 pandemic.

The source of the infections

WHO says it is investigating how a virus that is relatively rare in people got aboard a cruise ship.

The Argentine government’s leading hypothesis is that the Dutch couple who died contracted the virus during a bird-watching outing in the city of Ushuaia before boarding, according to two investigators. They spoke on condition of anonymity because they were not authorized to brief the media, with the investigation ongoing.