Health officials learned this weekend of the second hantavirus death, which killed a person who visited the park in June, spokesman Scott Gediman said in a statement.
There was one other confirmed case of the illness, and a fourth is being investigated.
Yosemite officials said the four visitors might have been exposed while vacationing at the park's Curry Village, and are warning those who stayed in the village's tent cabins from mid-June through the end of August to beware of any symptoms of hantavirus, which can include fever, aches, dizziness and chills. An outreach effort is under way to contact visitors from that period who stayed in "Signature Tent Cabins," which have more insulation and amenities than other tent cabins.
Federal health officials say symptoms may develop up to 5 weeks after exposure to urine, droppings, or saliva of infected rodents, and Yosemite advised visitors to watch for symptoms for up to six weeks.
Of the 587 documented U.S. cases since the virus was identified in 1993, about one-third proved fatal. There is no specific treatment for the virus.
After-hours calls to Yosemite officials seeking further details were not immediately returned Monday night.
Following the first death, which was reported earlier this month, state health officials advised anyone with symptoms to seek medical attention and let doctors know if they were camping in Yosemite. Officials said thousands of people visit the park every month, so it would be impossible to track everyone who had set foot in Curry Village.
Curry Village is the most popular and economical lodging area in the park, a picturesque assemblage of rustic cabins at the base of the 3,000-foot promontory Glacier Point.
Gediman told the San Francisco Chronicle that of the 408 tent cabins in the village, 91 are of the "signature" variety where the four cases had stayed, which feature more insulation and amenities than the others.
It was not clear how many people stayed in the cabins in the period in which park officials are warning visitors.
Gediman said contractors are working on the signature cabins to protect park-goers.
"They're doing everything they can to eliminate areas where mice can get into the cabins," Gediman told the San Francisco Chronicle. "This was never because the cabins were dirty, it was never because we didn't take care of them. This is just because approximately 20 percent of all deer mice are infected with hantavirus. And they're here in Yosemite Valley."
This year's deaths mark the first such deaths in park visitors, although two others were stricken in a more remote area in 2000 and 2010, officials said.