Mammoth Lakes, California

Cluster of three fatal hantavirus pulmonary syndrome (HPS) cases in residents of Mammoth Lakes, Mono County, California. All three residents had symptom onset in February 2025; the third death was confirmed by Mono County Public Health on 3 April 2025. The cluster is unusual on two counts: (1) it occurred in late winter / early spring, whereas Sin Nombre virus cases typically peak in late spring or summer; and (2) one of the three reported no mice in the home and Mono County Public Health Officer Dr. Tom Boo said "[None] engaged in activities typically associated with exposure, such as cleaning out poorly ventilated indoor areas." The cluster brought Mono County's documented cumulative HPS case total to 29 since 1993 — the highest of any California county. Sin Nombre virus is carried by the deer mouse (Peromyscus maniculatus), endemic across the Eastern Sierra Nevada.