MADRID (AP) — Health authorities across four continents Thursday were tracking down and monitoring passengers who disembarked a hantavirus-stricken cruise ship before its deadly outbreak was detected, and trying to trace others who may have come into contact with them since then.

In Argentina, a team of investigators has yet to leave for the southern town where they suspect the outbreak originated, officials from the country’s Health Ministry told The Associated Press on Thursday. The Argentine investigators suspect a Dutch couple may have contracted the virus while on a bird-watching trip before they boarded the cruise ship.

On April 24, nearly two weeks after the first passenger had died on board, more than two dozen people from at least 12 different countries left the ship without contact tracing, the ship’s operator and Dutch officials said Thursday.

Three passengers have died in the outbreak — a Dutch couple and a German national — and several others are sick. Symptoms usually show between one and eight weeks after exposure.

None of the remaining passengers or crew on the ship are currently symptomatic, the Netherlands-based Oceanwide Expeditions cruise ship company said Thursday.

The World Health Organization says the risk to the wider public is low. Hantavirus is usually spread by the inhalation of contaminated rodent droppings and isn’t easily transmitted between people.

“We believe this will be a limited outbreak if the public health measures are implemented and solidarity is shown across all countries,” said Dr. Abdirahman Mahamud, the WHO’s alert and response director on Thursday.

Three people, including the ship’s doctor, were evacuated Wednesday while the ship was near the West African island country of Cape Verde and taken to specialized hospitals in Europe for treatment.

The ship’s operator said Thursday that a total of 30 passengers — including the deceased Dutch man and his wife — left the vessel at St. Helena. The Dutch Foreign Ministry has put the figure at about 40. The company had not previously said publicly that dozens more people left the ship on April 24. The stop was the scheduled end of the cruise for some passengers.

It wasn’t until May 2 that health authorities first confirmed hantavirus in a ship passenger, the WHO says. That was in a British man evacuated from the ship to South Africa three days after the St. Helena stop. He was tested in South Africa and is in intensive care there.