Now we move on to the virus causing a health alert at one of the nation's most popular vacation spots, yosemite national park. Tonight the number of patients is growing and the virus is potentially... See More
Now we move on to the virus causing a health alert at one of the nation's most popular vacation spots, yosemite national park. Tonight the number of patients is growing and the virus is potentially deadly. Nearly 30,000 people could now be at risk. David wright is tracking it all for us. Reporter: In one of the most beautiful places on earth, a deadly disease is lurking. Yosemite rangers are warning every park visitor about an outbreak of hantavirus, spread by rodents. Actually had a mouse run across my head! Definitely scared me pretty good. Reporter: Already eight people are known to have contracted the illness. As of today, three have died -- in california, pennsylvania, and west virginia. And there may be more cases -- as far away as france. This outbreak appears to be unusual in terms of the number of people affected and certainly the number of deaths. Reporter: The outbreak was first traced to the popular curry village campsite, where rodents had burrowed into the insulation of tents. Officials now believe a second campsite in tuolomne meadows was also infected. Last week, warnings like these went out to 10,000 people who visited curry village since june. Overnight, notices went out to an additional 19,000 visitors. 29,000 recent park visitors in all. This is possibly the greatest cluster which has occurred in the united states that I am aware of. Reporter: Early symptoms, can take up to six weeks to appear, including fatigue, fever, and chills, giving way to coughing and breathing problems as the lungs fill with fluid. The worst part is -- there is no known treatment or cure. There is really nothing you can do preemptively if you were exposed to hanta virus. Reporter: For one of america's oldest and most popular national parks, a serious threat. Yosemite's majestic beauty attracts visitors worldwide. Now that could mean a worldwide epidemic. David wright, abc news, los angeles.
This transcript has been automatically generated and may not be 100% accurate.