FRESNO, Calif. (AP) — Up to 10,000 people who were guests in certain lodging cabins at Yosemite National Park might have been exposed to a deadly mouse-borne virus, park officials confirmed Friday as rangers handled a slew of calls from frightened visitors.

Park concessionaire Delaware North Co. sent letters and emails this week to nearly 3,000 people who reserved the insulated "Signature" cabins between June and August, warning them that they might have been exposed.

The cabins hold up to four people, and park spokesman Scott Gediman said Friday that means up to 7,000 more visitors might have been exposed to the virus that so far has killed two people and sickened four others.

Meanwhile, more than 1,000 calls a day are coming into Yosemite's new hantavirus hotline as visitors frightened about the growing outbreak of hantavirus pulmonary syndrome call seeking answers.

"We're reaching out and they are reaching out to us, and we are trying in every way shape and form to be transparent and forthright," he said. "We want to tell people this is what we know. The most important thing is the safety of park visitors and employees."

On Thursday, the California Department of Public Health confirmed that a total of six people have contracted the disease at Yosemite, up from four suspected cases earlier in the week.

Alerts sent to state and county public health agencies, as well as local doctors and hospitals, have turned up other suspected cases that have not yet been confirmed, according to the Centers for Disease Control and Prevention.

"Additional suspected cases are being investigated from multiple health jurisdictions," the CDC said in an advisory issued to health care providers.

The illness that begins as flu-like symptoms can take six weeks to incubate before rapid acute respiratory and organ failure.

There is no cure, and anyone exhibiting the symptoms must be hospitalized. More than 36 percent of people who contract the rare illness will die from it.

All of the victims confirmed so far stayed in the high-end, insulated "Signature" tent cabins in the park's historic Curry Village section between mid-June and early July.

Park officials worked quickly to disinfect all 400 of the Curry Village cabins when the outbreak first was detected earlier this month. When the outbreak was narrowed to the 91 double-walled insulated cabins, the California Department of Public Health ordered them shut down Tuesday.

Park officials said the double-walled design of those particular cabins 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 and carried on airborne aerosol particles and dust.

As the busy Labor Day weekend launches and word about the outbreak spread, some guests were cancelling lodging reservations at the park. But Gediman says others on waiting lists for hard-to-get accommodations are snapping them up.

The hantavirus outbreak occurred despite park officials' efforts to step up protections.

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 report revealed 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.

The park's 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."

The 91 insulated, high-end canvas cabins in the century-old Curry Village are new to the park. They were constructed in 2009 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.

Upon taking them apart for cleaning, park employees found evidence of mouse nests in the insulation.

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.

In 2011, half of the 24 U.S. hantavirus cases ended in death. But since 1993, when the virus first was identified, the average death rate is 36 percent, according to the CDC.

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The Yosemite hotline number is 209-372-0822. It's staffed from 9 a.m. to 5 p.m. Pacific Time.

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Jason Dearen reported from San Francisco.

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Reach Tracie Cone on Twitter: www.twitter.com/TConeAP

The infections spurred park officials to close 91 tent cabins at Curry Village in Yosemite Valley, where five of the six infections occurred. Gore said one of the infected people may have been in another area of the park.

"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 but can quickly affect the lungs. It can take up to six weeks to incubate.

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.

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.

In 2011, half of the 24 U.S. hantavirus cases ended in death. But since 1993, when the virus first was identified, the average death rate is 36.39 percent, according to the Centers for Disease Control.

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Dearen reported from San Francisco.

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    Half Dome (<a href="http://www.flickr.com/photos/amanda_walker/3139589261/" target="_hplink">AmandaWalker, Flickr</a>)

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    Giant Sequoia trees in Mariposa Grove (<a href="http://www.flickr.com/photos/tri_monopod/2495506392" target="_hplink">b.e.r.n.s., Flickr</a>)

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    Giant sequoias in Mariposa Grove (<a href="http://www.flickr.com/photos/palojono/3793407861/" target="_hplink">Palojono, Flickr</a>)

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    Theodore Roosevelt and John Muir stand on Glacier Point. (Library of Congress)

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    Yosemite Falls. The highest falls in North America. Upper falls, 2,425 feet, lower falls, 1,430 feet.(<a href="http://www.flickr.com/photos/docentjoyce/3673909650/" target="_hplink">docentjoyce, Flickr</a>)

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    Yosemite Valley (<a href="http://www.flickr.com/photos/27630470@N03/4015227797" target="_hplink">vl8189, Flickr</a>)

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    Grizzly Giant (<a href="http://www.flickr.com/photos/daveynin/6296590091/" target="_hplink">daveynin, Flickr</a>)

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    Hikers walk through the snow at the top of Yosemite Falls. (<a href="http://www.flickr.com/photos/mulsanne/4498889718/" target="_hplink">Mulsanne, Flickr</a>)

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    Three Brothers and the Merced River from Cathedral Beach. (<a href="http://www.flickr.com/photos/bumeister/5146841571/" target="_hplink">bumeister1, Flickr</a>)

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    El Capitan (<a href="http://www.flickr.com/photos/fortherock/3935179336/" target="_hplink">fortherock, Flickr</a>)

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    El Capitan, Cathedral Rocks and the Merced River. (<a href="http://www.flickr.com/photos/bumeister/5147448506/" target="_hplink">bumeister1, Flickr</a>)

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    A horse-drawn carriage passes through a tunnel in a redwood tree in Yosemite National Park, Calif. (U.S. Department of the Interior / AP)

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    Wawona Tunnel Tree, Mariposa Grove. (<a href="http://www.flickr.com/photos/11401127@N04/1519498714/" target="_hplink">phil virgo, Flickr</a>)

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    Climbers on El Capitan. (<a href="http://www.flickr.com/photos/parksdh/5182429949/" target="_hplink">D.H. Parks, Flickr</a>)

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    Yosemite National Park's Tuolumne Meadows is shown, Thursday, June 15, 2006, in Yosemite National Park, Calif. Looking east across the Tuolumne River you can see Lembert Dome and Mt. Dana. (Al Golub, AP)

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    WPA poster for Yosemite. (Library of Congress)

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    View of Half Dome from Glacier Point. (<a href="http://www.flickr.com/photos/matthigh/2066780234/" target="_hplink">mhlradio, Flickr</a>)

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    Fall colors in Yosemite National Park. (<a href="http://www.flickr.com/photos/dsleeter_2000/3063417514/" target="_hplink">dsleeter_2000, Flickr</a>)

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    The setting sun hits Horsetail Fall at just the right angle to light it up as if it's on fire, in Yosemite, Calif. This natural phenomena occurs for just two weeks in February and is reminiscent of the old firefall of burning embers that park employees pushed over Glacier Point to entertain guests until 1968. (Bethany Gediman, Yosemite National Park Service / AP)