The Andes Hantavirus Strain & What a Few Moments of Contact Can Do

In early April 2026, passengers aboard the MV Hondius began falling ill.

The Dutch-flagged expedition ship had departed Ushuaia, Argentina, on April 1, carrying 88 passengers and 59 crew members representing 23 nationalities. By early May, three people had died. Laboratory testing in South Africa confirmed the pathogen as a hantavirus, and the World Health Organization later identified the strain as Andes virus, the species responsible for most hantavirus pulmonary syndrome cases in South America.

Several countries are now coordinating an international response that includes case isolation, contact tracing, medical evacuation, and environmental investigation. The Hondius situation has drawn international attention not because hantavirus itself is new, but because of the specific strain involved.

Most hantaviruses are dead-end infections in humans: people contract them from rodents and do not pass them on. Andes virus is the exception. It is the only hantavirus on Earth with documented human-to-human transmission, and that single fact changes how outbreaks involving it must be handled.

To understand why this incident has prompted the response it has, it helps to understand what makes Andes virus different from every other hantavirus that has ever been studied.

A Hantavirus Unlike the Others

Hantaviruses are a family of rodent-borne viruses found on every inhabited continent. People typically become infected by inhaling aerosolized particles from the urine, droppings, or saliva of infected rodents, often in enclosed spaces where contaminated dust gets stirred up.

Different hantaviruses cause different illnesses, broadly grouped into two categories:

• Hemorrhagic fever with renal syndrome, which predominates in Asia and Europe.
• Hantavirus pulmonary syndrome, which is the form found in the Americas.

The WHO fact sheet on hantavirus describes the disease as rare but severe.

The Dead-End Pattern

In nearly every documented case worldwide, the chain of transmission stops at the first human host. A person who contracts Sin Nombre virus from deer mice in the American Southwest does not pass it to family members. A worker exposed to Seoul virus through rats does not transmit it to coworkers. The CDC's overview of hantavirus emphasizes this rodent-to-human pathway as the standard mode of infection.

Andes is different. Outbreaks in Argentina and Chile dating back to the mid-1990s have produced clear evidence that the virus can move from one person to another under the right conditions, with the infected person continuing the chain. That makes it unique among more than 20 known hantavirus species, and it is why public health authorities respond differently when Andes virus is suspected.

Origins in Patagonia

Andes virus was first identified in 1995, when researchers detected its genetic material in tissue from a patient in El Bolsón, a town in the foothills of the Argentine Andes. Cases were reported in Chile that same year.

The virus was named for the mountain range that runs through the region where it was found. Its primary source is the long-tailed pygmy rice rat (Oligoryzomys longicaudatus), a small rodent native to the temperate forests and grasslands of southern South America.

The virus has been studied for three decades. It is endemic across a wide portion of Argentina and Chile, with the Pan American Health Organization tracking cases throughout the Americas in routine epidemiological surveillance.

Despite this long history, Andes virus has not been a familiar name outside the public health and infectious disease research communities, in part because outbreaks have remained geographically concentrated and relatively small.

The Evidence for Person-to-Person Transmission

El Bolsón, 1996

The first signal that Andes virus could move between people came from El Bolsón in the second half of 1996.

Eighteen cases of hantavirus pulmonary syndrome appeared in residents and visitors to El Bolsón, Bariloche, and Esquel over a roughly three-month period. Two additional cases occurred in people who had contact with El Bolsón patients but had never visited the area themselves.

Five of the patients were physicians. Three of them had directly cared for an infected patient.

Investigators documented epidemiologic links between most of the cases and noted that local rodent populations were too sparse to plausibly account for the cluster. The findings were published by the CDC's Emerging Infectious Diseases journal in 1997.

In the years that followed, additional outbreaks added to the picture:

• 2014, Argentina: A cluster was traced through genomic and epidemiologic analysis, with the resulting study further confirming person-to-person spread under specific contact conditions.
• 2020, Chile: Researchers documented a different transmission route entirely. A mother passed the virus to her infant through breast milk, with the genome and proteins of the virus detected in cells from the breast milk itself. The findings suggested gastrointestinal infection as another viable route of person-to-person spread.

Epuyén, 2018-19

The most detailed picture came from the village of Epuyén in Argentine Patagonia, where an outbreak between November 2018 and February 2019 produced 34 confirmed cases and 11 deaths.

A research team that included scientists from Argentina's national laboratory, the U.S. Army Medical Research Institute of Infectious Diseases, and several universities reconstructed the chain of transmission using next-generation sequencing and contact tracing. They published their findings in the New England Journal of Medicine.

After a single introduction of the virus, the outbreak was driven by three symptomatic individuals who attended crowded social events, including a birthday party. Once isolation and quarantine measures were in place, the reproductive number dropped from above two to below one, and the outbreak ended.

What "Close and Prolonged Contact" Actually Means

Public health agencies consistently describe Andes virus transmission as requiring close and prolonged contact. That phrasing is accurate, but it is also incomplete in a way that matters for understanding risk.

The Narrow Infectious Window

The Epuyén investigation found something that surprised even the researchers studying it. People with the virus were most infectious during a brief window, with the highest viral shedding around the day they developed fever.

Within that window, transmission could occur after relatively short periods of close contact. The 68-year-old man at the center of the Epuyén outbreak attended a birthday party on the same day he ran a fever, and over the 90 minutes he was there, he infected at least five other guests. One of those infections occurred after only a few moments of contact, when he passed another guest on the way to the restroom.

What this means in practice is that the conditions for transmission are narrow but not exclusively about duration. The virus appears to require close physical proximity, but the exposure window itself is short, and it falls in a phase of illness when the infected person may not yet realize they are seriously sick.

The overall picture from three decades of study remains consistent: Andes virus does not behave like influenza or COVID-19. It does not move easily through casual encounters, public spaces, or fleeting interactions in well-ventilated environments. There is no evidence of sustained community transmission anywhere in the world.

But "close and prolonged contact" is shorthand for a more specific reality involving timing, viral shedding, and shared indoor space.

Why Confined Indoor Environments Change the Calculation

Cruise ships have figured prominently in modern infectious disease history because they combine several conditions that magnify transmission risk for almost any contagious illness:

• Passengers and crew share dining rooms, lounges, and ventilation systems for days or weeks at a time
• Cabins are often shared
• Medical facilities aboard are limited compared with land-based hospitals
• When illness emerges, the population aboard cannot easily disperse without potentially carrying pathogens to multiple countries

Beyond Cruise Ships

These same dynamics apply to a wider range of environments than cruise ships alone.

Other maritime vessels present similar conditions: confined quarters, shared air and surfaces, limited medical care, and crews that live in close proximity for extended periods. This includes cargo ships, fishing boats, research vessels, and naval ships.

Remote worksites present comparable risk profiles. Examples include:

• Offshore oil platforms
• Mining camps
• Oilfield worksites
• Antarctic research stations
• Forestry operations in rural endemic areas

On land, the same principles apply to a range of occupational and non-occupational settings. Healthcare facilities have repeatedly figured in hantavirus transmission, with the El Bolsón cluster including multiple physicians who became infected after caring for patients.

Other land-based settings that share the basic structural features allowing respiratory pathogens to spread more readily include:

• Long-term care facilities
• Dormitories
• Military barracks
• Prisons and shelters
• Households where multiple people share small living spaces for extended periods
• Indoor social gatherings in regions where the virus is endemic

None of this means that any of these settings is inherently dangerous with respect to Andes virus. The virus is not present in most of these environments most of the time. The point is that when an infected person enters a confined indoor space and reaches the early symptomatic phase, the conditions that make transmission possible exist.

The Illness Itself

From Mild to Critical

What distinguishes the disease is what happens next. The CDC's clinical overview describes the rapid progression from this prodromal phase to severe respiratory distress, often within a day or two of the patient's condition appearing to worsen.

Fluid accumulates in the lungs. Blood pressure drops. Patients can deteriorate to the point of requiring mechanical ventilation and intensive care within hours.

There is no antiviral treatment specifically approved for the disease. Care is supportive: oxygen, mechanical ventilation, careful fluid management, and, in the most severe cases, extracorporeal membrane oxygenation.

A High Case Fatality Rate

The case fatality rate is what gives Andes virus its reputation.

• WHO's risk assessment notes that hantavirus infections in the Americas carry a case fatality rate of up to 50 percent.
• PAHO surveillance for 2025 reported 229 cases and 59 deaths across the region, a fatality rate of roughly 26 percent.

By any measure, this is a disease that kills a substantial fraction of the people it infects, regardless of whether they were exposed through a rodent or another person.

What Scientists Still Don't Know

Three decades of research have established certain factors about Andes virus, while other questions remain unanswered.

Open Questions About Transmission

Researchers do not fully understand why person-to-person transmission occurs in some Andes virus outbreaks and not others. The 2018-19 Epuyén event was driven by what investigators called superspreaders: a small number of individuals whose viral shedding patterns produced disproportionate transmission.

The biological reasons some patients become superspreaders are not yet clear. The genetic features of the virus that enable human-to-human transmission, and whether those features are stable or evolving, are still being studied.

Open Questions About the Hondius Outbreak

For the Hondius outbreak specifically, important questions are still under active investigation. WHO has noted that the source of exposure has not been determined.

Investigators are working to establish:

• Whether the cases originated from rodent contact during the ship's itinerary, which included stops at remote sites such as Antarctica, South Georgia, Tristan da Cunha, and Saint Helena.
• Whether exposure occurred before passengers boarded in Ushuaia.
• Whether person-to-person transmission aboard the ship contributed to the cluster, particularly given the close contact between the first two confirmed cases.

Genetic sequencing of the virus from confirmed patients is ongoing, and that analysis will help determine whether the cases share a single source.

CIDRAP director Michael Osterholm, discussing the outbreak, noted that the typical incubation period suggests passengers may have been exposed before boarding. He also emphasized that this remains an isolated event and that nothing about the situation resembles the early stages of a pandemic-scale respiratory virus.

Why the Public Risk Remains Low

WHO has assessed the overall risk to the global population from this event as low, and it has not relaxed that assessment as the investigation has progressed.

The reasoning rests on what three decades of Andes virus research have established:

• The virus does not have a documented presence outside specific regions of South America.
• Its rodent reservoir (the animal species that naturally carries the virus) is geographically constrained.
• The transmission window between people is brief, and the contact conditions required are specific.
• There is no evidence anywhere in the published literature of sustained community spread.
• Past outbreaks involving person-to-person transmission have been controlled with conventional public health measures: case identification, isolation, contact tracing, and quarantine.

The Epuyén outbreak followed exactly that pattern, with the reproductive number dropping below one once those measures were in place.

The Hondius response reflects this same playbook, applied internationally because the passengers and crew represent nearly two dozen countries.

Authorities have:

• Advised passengers to remain in their cabins where possible.
• Recommended a 45-day symptom monitoring period.
• Coordinated medical evacuations for the most seriously ill.
• Initiated contact tracing for individuals who left the ship at earlier ports or on connecting flights.

What Changed with the MV Hondius

For most of its history, Andes virus has been a story told in medical journals and in the small towns of southern Argentina and Chile. The MV Hondius outbreak has changed that. A virus that has been studied, contained, and largely ignored outside its endemic range is now the subject of an international response involving multiple countries and contact tracing across continents.

This has not produced a new pathogen or a new threat. It has produced new visibility for one that already existed.