Up to 10,000 people in the United States may be at risk from a deadly virus after visiting Yosemite National Park in California.
Six visitors to the park have contracted Hantavirus, a rare lung disease for which there is no cure. Two of them have already died.
People become infected by breathing in dust contaminated by rodent droppings. Symptoms can take up to six weeks to show and one third of cases are fatal.
The outbreak of the virus thought to have been caused by mice nesting in the insulation of tents at a campsite in the Curry Village area at Yosemite.
The Centers for Disease Control and Prevention said about 10,000 visitors who stayed there between June and August and could be at risk of contracting the virus.
Doctors have been urged to report diagnosed cases of Hantavirus to state health agencies.
Nearly four million people visit Yosemite National Park annually and about 70% of them visit Yosemite Valley. However, the outbreak of the virus has not led to a wave of cancellations.
There were two cases of Hantavirus in a more remote area in 2000 and 2010, but the deaths this year are the first.