FRESNO, Calif. (AP) — A second person has died of a rare, rodent-borne disease after staying in one of Yosemite National Park's most popular lodging areas, prompting federal officials to step up efforts to locate and warn recent visitors.

"I know they are actively trying to notify people," said Capt. Charles Higgins, director of the Office of Public Health for the National Park System.

Federal epidemiologists learned over the weekend of the fatality. That case and another brings to four the number of people who have contracted Hantavirus Pulmonary Syndrome, which can be carried by dust particles that come into contact with the urine, saliva or feces of an infected deer mouse.

Earlier this month a man from Alameda County in the San Francisco Bay Area died and a woman from Southern California was sickened after staying in infected tent cabins in Yosemite's popular Curry Village, a family friendly area with the park's lowest-cost accommodations.

In each of the four cases, visitors stayed in the Curry Village "Signature Tent Cabins," canvas-sided lodging insulated against the elements. The four people known so far to have contracted the illness stayed around the same time in June.

Yosemite officials 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. Park officials warn anyone with these flu-like symptoms to seek medical help immediately. There is no specific treatment for the respiratory illness.

Federal health officials say symptoms may develop up to six weeks after exposure to urine, droppings or saliva of infected rodents. Of the 587 documented U.S. cases since the virus was identified in 1993, about one-third proved fatal.

Thousands of people visit the park every month, so it would be impossible to track everyone who had set foot in Curry Village, officials said.

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. Of the 408 tent cabins in the village, 91 are of the "signature" variety where the four cases had stayed, which feature more amenities than the others.

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

Park spokesman Scott Gediman said the Delaware North Company, which runs the park's lodging facilities, is working to shore up cabins to protect park-goers.

"There are rodents and some are infected and that's what happens," Gediman said. "This is a wilderness setting. It has nothing to do with the cleanliness of the cabins."

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

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Online:

CDC on hantavirus: http://www.cdc.gov/hantavirus