IS IT HANT-ING YOU?

Published in the May 9 2026 edition.

Hantavirus is once again making headlines.

That’s not unusual here. Mono County saw three confirmed deaths from the virus last spring. But recent reports of an infected cruise ship have pushed the disease back into the national spotlight.

Which begs the question, why does Mono County see hantavirus cases year after year, while most of California rarely does?

A new line of ecological and evolutionary research suggests part of the answer may lie in the region’s rodents and the unusual way their populations overlap here.

That was the focus of a recent talk at the Sierra Nevada Aquatic Research Laboratory (SNARL) by University of California, Berkeley PhD candidate Tommy Herrera, a mammalogist and evolutionary biologist studying the relationship between deer mice and hantavirus.

Out of roughly 2,000 mice sampled by his team across Mono County, about 80% tested positive for Sin Nombre virus, the primary strain of hantavirus in North America.

Hantavirus itself isn’t a single virus, but a group of related viruses carried by rodents. First identified in modern science near the Hantan River in South Korea in 1978, its history stretches back much further, with suspected outbreaks recorded as early as 960 A.D. during China’s Song Dynasty.

Globally, hantaviruses fall into two broad categories. In Europe and Asia, they tend to cause hemorrhagic fever affecting the kidneys, with relatively low fatality rates. In the Americas, however, strains like Sin Nombre attack the lungs, causing hantavirus pulmonary syndrome, a far deadlier illness with a fatality rate of roughly 35–50%.

For comparison, fatality rates for COVID-19 hover around 1%.

Even within the hantavirus family, risks vary. The Andes virus, likely recently identified in the cruise ship case, is one of the few known strains capable of human-to-human transmission. 

Sin Nombre virus, by contrast, spreads from rodents to humans, primarily through inhalation of aerosolized particles from dried urine or feces.

Its emergence in the United States dates back to 1993, when a cluster of sudden deaths among young, otherwise healthy people in the Four Corners region puzzled health officials.

At first, investigators from the Centers for Disease Control and Prevention suspected influenza or environmental toxins. Only after extensive testing did they identify a new form of viral hemorrhagic fever.

But while federal scientists searched for answers, local Indigenous communities already had one.

Navajo and Hopi oral histories had long warned of a connection between rodents and disease, particularly following wet years. Tribal elders pointed to earlier outbreaks, in 1918, 1933, and beyond, that followed periods of heavy precipitation.

The pattern held up in 1993. An El Niño event brought increased rainfall, leading to a surge in pinyon pine nuts, which are a key food source for deer mice. The rodent population exploded, and with it, the virus.

When researchers began listening to those accounts, the investigation ultimately accelerated.

Herrera’s work hinges on that principle, that to understand a disease you must fully understand the host.

In California, that means understanding deer mice, not as a single species, but as several genetically distinct populations. Northwestern, northeastern and southern lineages all converge in the Eastern Sierra, particularly in Mono County.

That overlap may create ideal conditions for viral exchange and evolution, increasing the likelihood of new variants emerging.

“What we see is that the Eastern Sierra is a hotspot,” Herrera said. “Not only for spillover, but for novel emergence of viruses.”

Environmental factors add to the risk. Snowmelt supports lush seasonal growth, boosting food availability for rodents. At the same time, the region’s dry climate allows viral particles to become airborne more easily once contaminated material dries out.

Together, those conditions may help explain why Mono and Inyo counties, which are home to just a fraction of California’s population, account for a disproportionate number of hantavirus deaths.

The good news is that the Sin Nombre virus is relatively fragile outside a host. Sunlight, disinfectants, and proper ventilation all break it down quickly.

Which means prevention is straightforward.

If you encounter rodent droppings indoors: ventilate the space for at least 30 minutes. Wear a mask. Saturate the area with a 10% bleach solution and let it sit for 10 minutes before wiping it up.

Avoid sweeping or vacuuming, which can send particles into the air.

With the right precautions, the risk drops significantly.