FILE: Deer mice in a nesting box.

It’s been nearly a year since three people died in Mammoth Lakes due to hantavirus, and the popular ski region is still taking precautions seriously.

Justin Caporusso, the Mono County public information officer, told SFGATE in a phone call last month that last year’s fatal exposures to hantavirus — which is spread in the Eastern Sierra through the droppings or saliva of infected deer mice — were “a complete seasonal anomaly.”

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“It mostly occurs in warmer months, so we often launch campaigns towards later spring,” he said. “So when it was still winter, that kind of threw us off as well.”

Three people died in the first few months of 2025 of hantavirus pulmonary syndrome, a rare disease caused by hantavirus infection, including one 26-year-old Mammoth Mountain Ski Area employee, as SFGATE previously reported. While there was never a definitive link found between the individuals, health officials found deer mice droppings in each of their workplaces. 

At the time of the deaths last year, Dr. Tom Boo, Mono County public health officer, explained that there was likely an increase in deer mice populations that year throughout the Eastern Sierra, which resulted in an increase in droppings. Mono County has had only 27 recorded cases of hantavirus since 1993.

Color-enhanced transmission electron micrograph of hantavirus particles. Hantavirus is a respiratory disease carried in wild rodents such as deer........

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