Rare South American Diseases Offer Clues on the Next Pandemic

By the time colleagues called Dr. Matías Lahitte, an Argentine infectious disease specialist, it was too late.

The patient they asked him to examine last month was a 65-year-old woman from Rosario, Argentina’s third-largest city. She had a high fever, muscle pain and fatigue. A blood test indicated her white-blood cell and platelet counts were dangerously low.

Less than a week later, she was dead.

Test results revealed that she had contracted Argentine hemorrhagic fever, or A.H.F., a rare but serious disease carried by rodents that both the United States and Soviet Union once researched as a potential agent for biological weapons.

“It is remarkable that it is not discussed more,” Dr. Lahitte said of the disease, which can cause internal bleeding, seizures and comas. “The illness is persistent and debilitating.”

The woman’s death is part of an surprising resurgence of the disease in Argentina, which after a decades-long dormancy is spreading to places it has previously never been seen. As human development, environmental degradation and climate change relentlessly fragment and alter animal habitats, scientists warn that zoonoses — diseases that jump from animals to humans — are becoming more frequent and widespread.

Climate change is nudging animals, like the rodents that carry A.H.F., into new areas and bringing previously separated species into contact with one another. A 2022 study estimated that more than half of known human pathogenic diseases could be aggravated by climate change. And human activities like agriculture and urbanization are pushing humans into closer contact with many disease-carrying animals.

“Because of all the changes in climate, and also changes in how humans react to climate, we are basically expanding the human footprint on places where it hasn’t been,” said Dr. Pranav Kulkarni, a researcher at the University of California, Davis, who studies rodent-borne diseases in South America.

About three-quarters of emerging infectious diseases in humans come from animals, according to the U.S. Centers for Disease Control and Prevention. Dr. Kulkarni hopes that research like his, which aims to predict how climate change and human activity affect the spread of disease, could help local health officials catch future outbreaks — before they metastasize into the next global pandemic.

“Any one of these vector-borne or animal-borne or rodent-borne diseases might be a good candidate for the next big thing,” he said.

Until recently, the Rosario patient’s case would have been an anomaly. Since A.H.F.’s discovery in the 1950s, it has almost exclusively affected male farm workers in Argentina’s fertile plains, outside urban areas. But the infected woman lived in the center of the city and was not a farmer.

Argentine health authorities counted nearly 60 confirmed cases of A.H.F. in 2022 — about three times the number five years earlier, according to government data. And there were outbreaks in 2024 and 2025, which collectively killed at least nine people.

A warming climate and land-use changes are already having an affect globally on well-known diseases like dengue, Chikungunya and yellow fever, which are carried by mosquitoes and have spread into new countries and regions in recent years.

Dengue, a flulike illness that can be fatal, is one of the fastest-growing infectious diseases in parts of South America, and scientists have linked its growth to environmental degradation and a warming planet. More than 10 million cases, and over 16,000 related deaths, have been reported to the World Health Organization since the beginning of 2024, with some estimates putting the number of cases far higher.

Scientists have also linked climate change to the spread of the Andes virus, a rare but dangerous hantavirus — another type of rodent-borne pathogen present in South America — that was linked to three deaths on a cruise ship earlier this year. That outbreak tested the nerves of a pandemic-weary world and spotlighted the kind of growing viral spillover risks that researchers like Dr. Kulkarni have been examining.

Increasingly, scientists are also tracking lesser-known diseases like A.H.F. that have emerged or surged back from relative obscurity.

Dr. Marcela Uhart, an Argentine veterinarian and researcher with U.C. Davis, remembers when A.H.F. was considered a widespread problem in her country during the 1970s and ’80s. Dr. Uhart’s parents had friends who died of the disease, and she grew up hearing radio advertisements asking survivors for blood plasma donations to help treat the infected.

The rollout of a vaccine helped bring case numbers down in the 1990s. And plasma treatments proved effective, bringing the fatality rate from as high as 30 percent down to 1 percent for patients who received prompt care. Outbreaks dwindled.

But a few years ago, Dr. Uhart noticed the numbers ticking back up.

“It was like, ‘Whoa, how is that happening?’” she said.

Cases weren’t just rising, they were cropping up in unexpected areas and infecting new types of people. After a truck driver tested positive in 2025, health officials took note.

“This used to be an illness of rural workers — it didn’t exist in cities,” said Dr. María Dolores Lucena, who has seen a recent increase in cases San Nicolás de los Arroyos, a growing port city outside of Rosario. “Now there are people who get infected without any such link, people who have never even left the city.”

Dr. Lucena said the increase in local cases was likely tied to the city’s expansion. Over the years, she has watched San Nicolás de los Arroyos stretch toward the port and along the river, with gated communities popping up near open fields in formerly rural areas.

Similar transformations have happened across Argentina, leading researchers to wonder whether land-use changes and climate shifts are driving the spread of disease.

Dr. Kulkarni, Dr. Uhart and other scientists investigated that hypothesis in several recent studies. In April, the researchers built models to predict the risk to humans from the movements of the rodents that carry South American arenaviruses, a cl*** of viruses that are typically spread by rats and mice.

Shifts in temperature, precipitation and seasonal weather patterns have made some regions less suitable for the rodents, while others became new potential hot spots, they found.

Unlike more transmissible diseases, such as Covid-19, many hantaviruses and arenaviruses in South America don’t spread easily between people. But researchers who study A.H.F. and its relatives hope to boost understanding of other pathogens that could pose international risks.

With the certainty of rising temperatures and other environmental factors that could cause diseases to proliferate, Dr. Uhart said health officials should try to get one step ahead of outbreaks — with the help of climate modeling — instead of responding once new cases have already appeared.

“We try to turn off the fire once it already is burning,” Dr. Uhart lamented. “Can you really predict the next emerging disease? You really can’t. But you can predict or understand better the risk factors.”