Federal and state health officials investigating the hantavirus outbreak at Yosemite National Park say it could take several weeks to determine how nine visitors contracted the illness – three of whom died.

However, there’s some evidence the park’s population of deer mice, which carries hantavirus, has grown significantly. Park officials have been trapping and killing deer mice for the past few weeks.  They said that while the percentage of those testing positive hasn’t changed, there are simply more deer mice, which could translate into a greater risk of exposure for people.

Health officials are looking at Curry Village, the popular campsite where all but one of the cases originated. They found deer mice in the insulated walls of some tent cabins. Ninety-one cabins are now closed indefinitely.

Symptoms of hantavirus mimic the flu, and a patient who waits to seek treatment can go from bad to worse – fast.  Why some people got sick and others did not remains a mystery.

“The cases have been (visitors from) distinct cabins, different cabins, different families, and along with that, we know there are people that stay with case patients in tents – patients that didn’t get sick,” said Danielle Buttke, a veterinary epidemiologist for the National Park Service.

This is not the first hantavirus outbreak at Yosemite, but given the cluster of infections, it is the most serious. There are usually just a handful of cases each year in the United States, and it’s particularly worrisome because there is no cure or vaccine.

One-third of hantavirus cases are fatal, Dr. Charles Chiu, an infectious disease specialist with the University of California, San Francisco, told Fox News.

Epidemiologists studying hantavirus said all the attention on the Yosemite outbreak could help patients in the future.

“It is my hope that this particular cluster of cases will drive more interest in developing a vaccine or drug that would be effective against this very deadly disease,” Chiu said.

Park officials are handing out fliers with information about how to avoid getting sick. Tips include staying away from deer mice and their droppings. The park is also emailing about 230,000 people around the world who stayed overnight at the park since early June, giving them information about the virus and how it is spread.

The park averages about four million visitors each year, and park attendance is down compared to last August – although last summer was a huge tourist season because of the large snowpack-fed gushing waterfalls.

Now that the falls are dry, visitor numbers are on par with 2010 and 2009 figures, park officials said; however, park rangers and staff members have noted the scare has caused some people to cancel their hotel reservations.