Image caption The warning applies to visitors who stayed in the cabins at Curry Village from mid-June onward

Thousands of people could be at risk from a deadly virus in California's Yosemite National Park that has already claimed two lives, officials say.

Four other cases of Hantavirus, a rare lung disease, have been reported.

The park said it is getting about 1,000 calls per day from frightened visitors on its Hantavirus hotline.

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.

The 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, stuffy spaces.

The disease can also spread if people touch or eat contaminated substances, or are bitten by an infected animal.

The first death was reported in August. One of those who died was a 37-year-old man from the San Francisco Bay area.

Extreme breathing difficulty

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.

Media playback is unsupported on your device Media caption Lola Russell of the Centers for Disease Control and Prevention (CDC) says they are working with various agencies to tackle the outbreak

The CDC added that they were looking into suspected cases of the disease in "multiple health jurisdictions".

They also urged doctors to report diagnosed cases of Hantavirus to state health authorities.

The park has contacted about 3,000 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.

Earlier this 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.

"Right now it's normal numbers for Friday," Yosemite spokeswoman Kari Cobb said.

"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.

"It's still summer and a holiday weekend. It's still the summer crowds," she said.

Nearly four million people visit Yosemite National Park annually and about 70% of them visit Yosemite Valley, where Curry Village is located.

The park has seen two other cases of the hantavirus in a more remote area in 2000 and 2010, but this year's deaths were the first.