Thousands of visitors to Yosemite National Park, one of America's top tourist attractions, have been warned to watch for symptoms of a rare rodent-borne disease which has already killed two people.

Health officials learned this weekend of the second death, a person who visited the park in June, a spokesman said.

Another case of the illness, called hantavirus, has been confirmed, and a fourth is being investigated. The first death was reported earlier this month.

Yosemite said the visitors might have been exposed while staying at the park's Curry Village, and they warned those who used 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.

Of the 587 documented US cases since the virus was identified in 1993, about one-third proved fatal. There is no specific treatment.

Federal health officials say symptoms may develop up to five weeks after exposure to urine, droppings, or saliva of infected rodents, and Yosemite advised visitors to watch for symptoms for up to six weeks.

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. It was not clear how many people stayed in the cabins in the period for which park officials issued the warning.

The cabins are now being altered to protect park-goers.

"They're doing everything they can to eliminate areas where mice can get into the cabins," a spokesman said.

"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% of all deer mice are infected with hantavirus. And they're here in Yosemite Valley."

This year's deaths mark the first such deaths among park visitors, although two others were affected in a more remote area in 2000 and 2010.