Countries around the world are preparing to deal with the more than 140 passengers and crew members on board a hantavirus-stricken cruise ship headed for the Canary Islands.

The vessel is expected to reach the Spanish island of Tenerife, off the coast of West Africa, early Sunday.

At least three passengers have died, and several other people have been infected.

Hantavirus is usually spread by the inhalation of contaminated rodent droppings and isn’t easily transmitted between people. Some scientists believe the Andes virus implicated in the cruise ship outbreak may be able to spread between people in rare cases. But the World Health Organization says the risk to the wider public from the outbreak is low. Symptoms usually show between one and eight weeks after exposure.

Authorities and the cruise operator have been providing updates, but some key information is still lacking.

Here’s what we don’t know:

Where the outbreak originated

Argentine investigators suspect a Dutch couple may have first contracted the virus while on a bird-watching trip before they boarded the cruise ship in Argentina on April 1. But no organization has confirmed where or how they acquired the disease.

Argentina’s Health Ministry has zeroed in on the nation’s southernmost town, Ushuaia. Officials plan to travel there in the coming days, according to a written statement to The Associated Press.

What happens next to the remaining passengers

Spanish authorities are preparing to receive the remaining passengers and crew members on Tenerife. Officials said Friday that passengers will be evacuated in small boats to buses only once their repatriation flights are ready to take them.

The United States agreed to send a plane to the Canary Islands to pick up its citizens, as will the British government. American passengers will be brought to a dedicated biocontainment and quarantine unit in Nebraska for assessment, officials said Friday.

Other countries have not yet made their plans public, and it is not clear how long boat passengers will have to wait for their flights.

Spain has requested medically equipped planes for passengers experiencing symptoms, Virginia Barcones, the country’s head of emergency services, said Friday.

How many people may have been exposed

Cruise operator Oceanwide Expeditions and Dutch officials said Thursday that more than two dozen people from at least 12 different countries left the ship at the remote island of St. Helena in the South Atlantic on April 24.

They included a Dutch woman who disembarked with her husband’s body. He was the first passenger to die, but it wasn’t until May 2 that health authorities first confirmed hantavirus in a ship passenger.

The delay left countries scrambling to track the passengers who got off the ship some two weeks earlier.

The passengers included a resident of the remote island of Tristan da Cunha who has been hospitalized with symptoms of hantavirus, according to the British Foreign Office.

Stephen Doughty, the U.K. minister of overseas territories, said in a message to the British overseas territory that his thoughts were with “the islander currently in hospital and their spouse who is isolating.”

The whereabouts of all of the passengers

Many of the passengers who disembarked at St. Helena traveled on to other countries, including the Dutch woman whose husband died on board. She flew to Johannesburg then briefly boarded a plane preparing to fly to Amsterdam. She was removed because she was too ill to travel, and later died.

South African and Dutch authorities are trying to trace the whereabouts of anyone who had contact with the woman during her travels. A flight attendant who had contact with her has tested negative for hantavirus after reporting symptoms.

Some governments, like the United Kingdom, have confirmed the whereabouts of their citizens who left the boat. However, U.K. officials do not know or have not made public how many others they have come into contact with since.

In the U.S., some state officials said they were monitoring a small number of residents who were on the ship and already went home. None has symptoms.