Park officials said the double-walled design of the 91 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.
Over the past three weeks, two people have died of hantavirus pulmonary syndrome after staying in one of the so-called "Signature" cabins at Curry Village in Yosemite Valley. Another person is confirmed ill and one more likely has the virus that has killed 36 percent of the people it infects.
Mike Gauthier, Yosemite chief of staff, said the design of the luxury cabins that are new to the park allowed for rodent infestation.
"We just weren't aware that design would lead to it," he said.
The illness begins as flu-like symptoms but can quickly affect the lungs. It can take up to six weeks to incubate. All four people who fell ill stayed in the tent cabins in June, 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 California Department of Public Health 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. The report said park officials should take steps to prevent mice from entering areas where people sleep.
"Inspections for rodent infestations and appropriate exclusion efforts, particularly for buildings where people sleep, should be enhanced," it said.
"We worked with Yosemite to evaluate risk and make recommendations to reduce the possibility of transmission to people," added Vicki Kramer, chief of the vector borne disease section of the health department. "That included reducing the number of mice, and excluding them from structures."
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.
"Those cabins were all immediately investigated and cleaned when initial reports came out," Gauthier said. "But we want to be extra sure and not take any chances."
Yosemite's hantavirus plan also calls for awareness training of park employees and prescribes protective measures and equipment to reduce exposure.
"That's been a clear part of our messaging. My understanding is we did implement all of the measures of those recommendations," Gauthier said.
"Yosemite, to their credit, has taken quite a few steps to address this," Kramer said. "But it's a wilderness area and these buildings aren't going to be tight. It's impossible to get rid of the deer mice, so there is going to be some risk to being in a wilderness area."
Meanwhile, the park sent warning emails and letters on Wednesday to another 1,000 people who stayed in tent cabins in Curry Village, after officials found that a computer glitch had stopped the notices from going out with the original 1,700 warnings on 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.
Most of the nearly 600 cases reported since 1993 have been in New Mexico, Colorado, Washington, Arizona and California. Most often they are isolated, so having this cluster of cases from a small area in Yosemite has perplexed public health officials.
The federal government has two epidemiologists working in the park. They are trapping mice and rodents in an effort to determine how much of the population carries the virus and to see whether there are more mice in Yosemite Valley this year than in other years.
Kramer warned people never to sweep or vacuum mouse droppings. Instead spray them with a mixture of bleach and water then wipe it up with paper towels or a mop.