Some recent visitors to Yosemite National Park have expressed concern that park employees allowed them to be potentially exposed to a deadly rodent-borne disease, even after workers knew previous visitors had contracted the disease.

Eight people are known to have contracted hantavirus pulmonary syndrome between June and mid-August, and three of them died, Yosemite reported last week. The dead include a Kanawha County, W.Va., resident who stayed at Yosemite in June and died within two weeks of the trip.

All but one of the ill stayed in a set of 91 cabins in the park's Curry Village area. The eighth visited several High Sierra camps in another part of the park, had mild symptoms and is now recovering, according to Yosemite.

Hantavirus is contracted by breathing in small viral particles released into the air from the urine or droppings of infected deer mice or other rodents. There is no known cure, and about 38% of those who contract the disease die.

The incubation period can range from six days to six weeks. Symptoms include fever and headache, but the telltale sign is shortness of breath, or "the impression that you have something in your lungs when you're breathing," said Pierre Rollin, an outbreak investigator at the Centers for Disease Control and Prevention.