The underlying causes of this summer's Hantavirus outbreak in Yosemite National Park are still unclear. This short summary article outlines the current state of affairs: Scientists hunt for cause of hantavirus outbreak at Yosemite
Much about the hantavirus remains a mystery. Sunlight kills it, and in a laboratory setting, it has remained viable for about 48 hours, Buttke said, although it may be able to live a few days beyond that. Experts believe an area must have an active mouse infestation for people to become infected.
Possible explanations include: chance, year-to-year weather variations, changed mouse populations, changed predator populations, particular aspects of cabin usage in Yosemite valley, changes in contact with people, and changes in the virus itself.
Of all those, while the last one is the scariest, it's also the least likely; the other explanations are far more likely.
Update: Steve Loughran points me to this report from the European Center for Disease Prevention and Control: Epidemiological update: Hantavirus outbreak at Yosemite Park
According to ECDC‘s risk assessment, the risk of infection is low as only eight cases have been confirmed so far among American visitors and none among European visitors, although several thousand people have been in the area.
Meanwhile, the US CDC is maintaining this resource with additional information and details: Outbreak of Hantavirus Infection in Yosemite National Park.
The chance of contracting Hantavirus in Yosemite National Park is extremely low, and doesn't appear to be any different from the chance of contracting the disease elsewhere in the rural western United States.
But the disease itself is mysterious and deadly, and worthy of continued study. My mother, who spent several decades living in the high desert of eastern California, recalls:
A couple of people at China Lake died from it shortly before we retired. Namely, a couple of old men, retired for many years, members of the writer's club that I joined for awhile. They bought some stuff from the Navy auction, and opened the sealed boxes and apparently inhaled virus-laden air from the box. Scary.
I'll keep my eyes open for updates on the research. In the meantime: go to Yosemite; enjoy the views; experience the wonders. It's too lovely a place to miss.