An additional 12,000 people who stayed at a remote campsite in California's Yosemite National Park are at risk from the deadly hantavirus, officials warn.
The warning comes as a West Virginian who visited the park in June became the third person to die from the virus.
Eight cases have been confirmed so far. There is no known cure for the virus, spread by infected rodent droppings.
Symptoms can take up to six weeks to show and one third of cases are fatal.
Yosemite National Park said it was getting about 1,000 calls per day to its hantavirus hotline from frightened visitors.
The hantavirus virus is carried in rodent faeces, urine and saliva. When it dries out and mixes with dust, it can be inhaled by humans, especially in small spaces.
The disease can also spread if people touch or eat contaminated substances, or are bitten by an infected animal.
Extreme breathing difficulty
The first death was reported in August. One of those who died was a 37-year-old man from the San Francisco Bay area.
Yosemite says that out of the eight cases linked with the park, five remaining individuals are either improving or recovering.
The outbreak of the virus at Yosemite is thought to have been caused by mice nesting in the insulation of tents at a campsite in the Curry Village area of the reserve.
About 10,000 visitors stayed at the campsite between June and August and could be at risk of contracting the virus, the Centers for Disease Control and Prevention (CDC) said.
The park service more recently extended its warning to 12,000 people who had stayed or were planning to stay at the High Sierra Camps from earlier this summer through mid-September, bringing the total number possibly exposed to 22,000.
One non-fatal case was linked to a stay at the High Sierra Camps in July.
The CDC has urged doctors to report diagnosed cases of the hantavirus to state health authorities.
The park has contacted several thousands of groups of visitors warning them to seek medical advice if they experience flu-like symptoms, including headache, fever, shortness of breath, muscle ache and cough.
Severe cases can lead to extreme breathing difficulty and death.
Last week, park officials closed all 91 "signature" cabins after finding deer mice, which carry the virus, nesting between the double walls of the luxury tents.
But they added that the outbreak of the virus had not led to a wave of cancellations.
"There have been cancellations, but it would be grossly overstated to say they're cancelling en masse. There's quite a bit of people out there still," Yosemite spokeswoman Kari Cobb said.
The park saw two other cases of the hantavirus in a more remote area during 2000 and 2010, but this year's were the first known deaths.