"Our investigation is trying to determine which area of the park that person visited," she said.
Over the past three weeks, two people have died of hantavirus pulmonary syndrome after staying in cabins at Curry Village in Yosemite Valley.
Park officials said the double-walled design of the cabins that were closed Tuesday made it easy for mice to nest between the walls. The disease is carried in the feces, urine and saliva of deer mice and other rodents.
The illness begins as flu-like symptoms, including including headache, fever, muscle ache, shortness of breath and cough. Initial symptoms may appear up to six weeks after exposure and can lead to severe breathing difficulties and death.
Although there is no cure for hantavirus, treatment after early detection through blood tests can save lives. The virus, which has never been known to be transmitted between humans, kills 38 percent of those it infects.
"The earlier it's caught and supportive care is given, the better the survival rate," said Dr. Vicki Kramer, chief of vector-borne diseases at the state Public Health Department.
Dr. Charles Chiu, an infectious disease specialist at the University of California, San Francisco, said he made a habit of airing out his tent-cabin before occupying it as a precaution against possible virus-carrying dust particles when he stayed in Curry Village a few years ago.
Even Chiu said he was surprised to learn that a hantavirus had killed two people and stricken others who slept in the same structures this summer.
"It wasn't something even I had thought of at the time," Chiu, who studies hantavirus, told Reuters.
Five of the people who fell ill are known to have stayed in the tent cabins in June or July, and warnings have gone out to visitors who stayed in Curry Village in June, July or August.
The hantavirus outbreak occurred despite efforts by park officials to step up protection efforts last April. A 2010 report from the state health department warned park officials that rodent inspection efforts should be increased after a visitor to the Tuolumne Meadows area of the park fell ill.
The new hantavirus policy, enacted April 25, was designed to provide a safe place, "free from recognized hazards that may cause serious physical harm or death."
It came after the state report revealed that 18 percent of mice trapped for testing at various locations around the park were positive for hantavirus.
"Inspections for rodent infestations and appropriate exclusion efforts, particularly for buildings where people sleep, should be enhanced," it said.
Melanie Norall, of Palo Alto, California, is monitoring her 8-year-old daughter's every sniffle. They stayed in a cabin outside Yosemite's north entrance at the end of July and awoke to mice scurrying and eating nuts out of their luggage.
In 2009, the park installed the 91 new, higher-end cabins to replace some that had been closed or damaged after parts of Curry Village, which sits below the 3,000-foot Glacier Point promontory, were determined to be in a rock-fall hazard zone.
The new cabins have canvas exteriors and drywall or plywood inside, with insulation in between. Park officials found this week when they tried to shore up some of the cabins that mice had built nests in the walls.
The deer mice most prone to carrying the virus can squeeze through holes just one-quarter-inch in diameter. They are distinguished from solid-colored house mice by their white bellies and gray and brown bodies.
The park sent warning emails and letters Wednesday to another 1,000 people who stayed in tent cabins, after officials found that a computer glitch had stopped the notices from going out with the original 1,700 warnings Monday. The warning says anyone with flu-like symptoms or respiratory problems should seek immediate medical attention.
"The message should not be you should stop camping. The important thing is general awareness of this disease and to avoid wild rodents in general," said Chiu.
Intellpuke: This article is a compilation of reporting by correspondents for NBC News, the Associated Press and Reuters news agency; you can read the complete article in context here: vitals.nbcnews.com/_news/2012/08/31/13591500-cdc-10000-at-risk-of-hantavirus-in-yosemite-outbreak?lite