The Montana Department of Public Health and Human Services have confirmed cases of hantavirus pulmonary syndrome in both Cascade and Flathead counties.
The agency did not release any information on those with the illness, including whether they were hospitalized or had recovered.
The cases bring to 35 the number reported in Montana since 1993. There have been three so far this year.
Hantavirus cases often increase as people begin to clean their garages and sheds in the warmer spring weather. Cleaning can disturb nesting materials contaminated with dried saliva, urine, or droppings from infected deer mice, which are inhaled, causing exposure to the virus.
The illness usually starts with flu-like symptoms, including fever, sore muscles and headaches. It progresses to a severe shortness of breath. Early diagnoses and treatment increase the likelihood of full recovery.