but why some people got sick and others didn't remains a mystery. >> the cases have been different cabins, different families. >> 23,000 campers may have been he can posed to the virus. epidemiologists want to know if the virus first identified in 1993 has mutated. >> to understand how it has evolved and the mechanisms by which viruses such as hantavirus can jump from animals to humans. >> reporter: live at yosemite there is evidence that the deer mice population in the park is growing. that could explain the outbreak. park attendance is down but there are still a lot of visitors here. most of them tell us they appreciate the park's education outreach efforts and say they