There was little known about the Andes strain of hantavirus before an outbreak at a birthday party eight years ago.

The deadly strain is again under the magnifying glass after an outbreak on luxury cruise ship, the MV Hondius, infected eight people, killing three people and leaving a fourth person fighting for life.

The four Australians and one permanent resident onboard are expected to land in Australia on Tuesday, and while global health experts insist hantavirus is “not COVID”, the world is wondering exactly what is known about how the virus spreads.

Health authorities raced to answer similar questions in southern Argentina in 2018, after nearly three dozen people fell gravely in the tiny village of Epuyen. By the end of the outbreak, 11 patients had died.

Their illness, which caused many to be admitted to intensive care for pneumonia and severe breathing problems, was caused by the Andes virus, a strain of rodent-carried hantavirus capable of being transmitted from person to person.

Dr Gustavo Palacios, a microbiologist at the Icahn School of Medicine in New York, helped piece together how the virus moved between humans when he was the director of the US Army Medical Research Institute of Infectious Diseases Centre for Genome Sciences.

“There is very limited experience handling this virus,” Palacios said.

He puts a rough “ballpark number” of total Andes virus human transmission as “300 cases in history” and about 3000 Andes cases overall.

Palacios is now also one of the experts advising on the ongoing cruise ship outbreak.

Based on their investigation of the Epuyen outbreak, which involved three separate superspreader events, Palacios said the window for transmission of the Andes virus appears to be short, about a day.

People are at their peak of infectiousness on the day they develop a fever.

But the study also found that the virus could be passed relatively easily during this window, after periods of only brief proximity to someone else.

Birthday party superspreader event

The researchers were able to show that the patient believed to have kickstarted the Epuyen outbreak, a 68-year-old man, had attended a birthday party with about 100 other people.

He attended the birthday party the same day he ran a fever.

He infected five others during the 90 minutes he was at the party, including two people sitting roughly a foot from him at the same table and two people who were sitting just over a metre away from him at neighbouring tables.

The fifth person to catch the virus crossed paths with the patient only briefly on their way to the restroom.

Another complication with the Andes virus is its long incubation period, meaning the time between a person’s exposure to the virus and when they first begin to show symptoms.

The long interval makes it particularly difficult to track down people who may have been exposed.

Although all five patients were exposed at the birthday party, they didn’t start to show symptoms for another two to three weeks.

The second patient in the outbreak, a 61-year-old man described as having an active social life, infected six others before he died, 16 days after first showing symptoms.

His wife, who attended his wake with a fever, infected 10 others, who all became sick between 17 and 40 days after attending that event.

An additional 12 people were infected after contact with previously infected patients.

Usually ‘a dead-end infection’

The index case — the first documented case — in the Epuyen outbreak is believed to have been infected near his home.

In Argentina, the Andes virus is carried by long-tailed pygmy rice rats, which are common in agricultural areas and can live around houses.

Rodents are known to harbour hantaviruses around the world, including in the US Southwest.

Humans are typically infected through contact with their urine, faeces or saliva, particularly when when the material is disturbed and becomes airborne, posing a risk of inhalation. This can happened when the virus becomes aerosolised during cleaning.

Recently, hantavirus made news in the US in 2025 after an autopsy determined that Betsy Arakawa, the wife of actor Gene Hackman, had died of the virus.

In most cases, hantaviruses results in what’s called a dead-end infection: A human gets infected after contact with animal droppings but doesn’t pass it on to anyone else.

Andes virus is an exception, however. It can spread between people, giving it the potential to spark outbreaks.

While the World Health Organisation (WHO) says the threat posed by the current outbreak on the cruise ship Hondius is low, WHO has classified hantaviruses as emerging priority pathogens with high potential to spark international public health emergencies because of how serious these infections can be.

Hantavirus infection can be lethal in up to 40 per cent of cases.

A limited window for spread

In the Epuyen outbreak, more than 80 healthcare workers were exposed to patients with symptoms, but none were directly infected themselves, though very few used any personal protective equipment.

There were two infected healthcare workers at the local rural hospital, a smaller facility, which may have been the first to see sick patients.

The limited spread among healthcare workers in the Epuyen outbreak speaks to the short window of time that a person may be infectious, experts said.

“This is not COVID. This is really not COVID. It’s not even influenza,” National Institute for Communicable Diseases in South Africa’s former deputy director Dr Lucille Blumberg said.

Of the cruise ship outbreak, she said: “It’s an unusual person-to-person event, and it might have happened because, perhaps, of a closed environment on a ship.”

Blumberg was consulted on Friday about the deaths related to the Hondius and another seriously ill passenger who had been on a different part of the ship, who was medically evacuated to Ascension Island, a British territory that sits in the Atlantic Ocean.

She said they would be following the passengers on the Hondius closely. They will each need to be monitored for at least 45 days, she said.

Contact tracing is underway for people who were on flights with ill passengers from MV Hondius. Oceanwide Expeditions, the cruise operator, said it is still working on the details of who embarked and disembarked from the ship since March.

“We expect to share details on that in the coming days,” Oceanwide Expeditions spokesperson Piet Hein Coeberg said.

“People come off and on at the ports,” Blumberg said. “They don’t stay for the whole voyage.

“I think we’ll see other cases,” she said.

A floating outbreak

Many of the passengers were serious birdwatchers who had been on expeditions in South America before joining the cruise, Blumberg said.

For that reason, avian influenza was one of her initial guesses as to the cause of the illnesses. She also suspected that people might have legionella infections, which can cause pneumonia.

After two rounds of tests were negative for those and other suspected pathogens, Blumberg said she called the lab at the Centre for Emerging Zoonotic and Parasitic Diseases at the National Institute for Communicable Diseases and told them to test for hantavirus.

After it was positive, she called the hospital that had treated the ill wife of the first passenger who died on the ship, and asked whether it had saved any test tubes of her blood. It had, and that patient, too, tested positive posthumously for hantavirus.

By Monday, gene sequencing determined that it was the Andes strain.

Blumberg said that for herself and her colleagues, it has been a round-the-clock effort; she was up at 4am on Wednesday.

She said they are actively working on tracing the contacts of patients who were evacuated to South Africa for medical care.

They are also working on sequencing the entire genome of the virus, which should help pinpoint where it came from and whether it has developed new mutations.

There has been global cooperation from the scientific community, all spearheaded by WHO, she said, and the international group working on the outbreak has already held three calls.

“We really have almost no experience with Andean (hantavirus),” Blumberg said.

Other infectious disease experts, such as Vanderbilt University preventive medicine professor Dr William Schaffner, say the situation on the MV Hondius does not have them worried, but they are very interested.

“I am transfixed,” Schaffner said. “It’s an extraordinarily unusual circumstance where there’s hantavirus infection on a boat, and I’m even impressed that they’ve made this diagnosis.

“It’s serious, and for us, scientifically, it has all these other curiosities about location and behaviors of new hantavirus variants.

“So there are lots of scientific issues, there are public health issues, there are issues of, how do you deal with seriously ill people on a cruise ship who have a communicable disease in the middle of the ocean?”