Three guests are dead as a suspected hantavirus outbreak spreads on board a cruise ship currently in the Atlantic Ocean, while three others are ill, according to an announcement from the World Health Organization (WHO) on Sunday.

The suspected outbreak occurred on the MV Hondius, a Dutch-flagged passenger ship that left Argentina several weeks ago. Its operator, Oceanwide Expeditions, said it is currently handling a “serious medical situation” onboard the vessel.

“WHO is aware of and supporting a public health event involving a cruise vessel sailing in the Atlantic Ocean,” the organization said in a statement on Sunday afternoon.

“To date, one case of hantavirus infection has been laboratory confirmed, and there are five additional suspected cases. Of the six affected individuals, three have died and one is currently in intensive care in South Africa.”

Hantavirus is a viral condition resulting most often from contact with the urine or feces of infected rodents, most often wild mice and rats. It has a mortality rate ranging from 30 to 50 percent, making it among the most lethal viruses in the world.

“While rare, hantavirus may spread between people, and can lead to severe respiratory illness and requires careful patient monitoring, support and response,” the WHO wrote.

South Africa’s Department of Health has identified the first deceased passenger as a 70-year-old man. He died on the ship, and his body was later removed in the British territory of Saint Helena in the South Atlantic.

His wife collapsed at an airport in South Africa while trying to take a flight home to the Netherlands, and later died in the hospital, the department told The Associated Press.

The department said the patient who was transferred from the ship to intensive care in Johannesburg, South Africa, is a British national who fell sick after the ship left Saint Helena.

The vessel, destined for Spain’s Canary Islands, is currently off the coast of Cape Verde. Oceanwide Expeditions, the tour operator, said local authorities there had not given permission for anyone to disembark the vessel as of May 3.

Two crew members onboard need urgent medical care, and while local officials have assessed their condition, no decision has been made by the local health officials on whether to transfer them to medical care, it added.

Dutch authorities have agreed to lead joint efforts to arrange the repatriation of the two crew members from Cape Verde to the Netherlands. The plan also includes the body of one deceased guest and another guest who had close contact but showed no symptoms, the company said.

“We are in close contact with those directly affected and their families and are providing support where possible,” it said in a statement. “The health and safety of all passengers and crew is our highest priority.”

It remains unclear how many people were on board the ship. South Africa’s Health Department said around 150 tourists were on board at the time of the outbreak.

Oceanwide Expeditions did not respond to a request for comment by publication time.

Most strains of the hantavirus don’t spread from person to person; however, one South American strain, known as the Andes virus, has defied this trend in the past, causing a hantavirus outbreak in Epuyén, Argentina, in 2018 to 2019.

The Associated Press contributed to this report.