A health warning over a killer virus is being sent to nearly a quarter of a million people who have stayed in Yosemite national park in California since June.
The mouse-borne hantavirus is now known to have infected nine people - three of whom have died, the BBC reports. There is no known cure.
Officials at the California park insist their letters to more than 230,000 people are purely precautionary.
Deer mice which spread the infection have increased in number and now the rodents are being trapped and killed.
Yosemite initially sent a warning last month to 1700 campers who stayed at a specific campsite, before gradually expanding that figure as infections emerged elsewhere.
Most of the cases involved guests at the Signature cabins in Curry Village, but one case involved someone who had stayed at multiple High Sierra camps in wilderness areas.
A ninth person is now recovering after being stricken with the disease following a visit to Yosemite in early July, said National Park Service spokesman John Quinley on Thursday.
It can take six weeks for the emergence of early symptoms such as aches and fever, or in half the cases headaches and sickness. It then moves into the lungs.
One third of cases are fatal, but identifying it early through blood tests increases a person's chance of survival.
The hantavirus 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 first death was reported in August. One of those who died was a 37-year-old man from the San Francisco Bay area.
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.
