Yosemite National Park is warning 1,700 visitors who stayed in some of its tent cabins this summer that they may have been exposed to a deadly virus.
Hantavirus pulmonary syndrome has been blamed for the deaths of two campers who stayed at the Californian park.
The disease can be carried in the urine, saliva and faeces of infected deer mice, and symptoms can appear as late as six weeks after exposure.
Two other infected campers were expected to survive.
The first death was reported earlier this month, and one of the victims was identified as a 37-year-old man from the San Francisco Bay area.
Fever and dizziness
There is no specific treatment for the hantavirus, which has a fatality rate of 30%.
The National Park Service, which runs Yosemite, extended the warning to visitors who stayed in the 408 canvas and wood cabins in Curry Village from mid-June onward.
They have been advised to be watch out for the symptoms of hantavirus, which include fever, aches, dizziness and chills.
"We are encouraging anyone who stayed in Curry Village since June to be aware of the symptoms of hantavirus and seek medical attention at the first sign of illness," a statement said.
Officials said they were working with contractors to clean and inspect the cabins.
"This is a serious public health issue and we want to be transparent, but at the same time we don't want people to alter their plans because we are taking the necessary precautions," said park spokesman Scott Gediman.
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.