Some 10 000 visitors to California's Yosemite National Park could have been exposed to a deadly virus that kills one in three victims and cannot be treated.
So far, six cases of the rare hantavirus pulmonary syndrome (HPS) have been confirmed - two of whom have died - while a "multiple" number of other suspected cases of the rodent-borne disease are being investigated.
Yosemite authorities closed down the 'Signature Tent Cabins' earlier this week at Curry Village, a popular lodging area in Yosemite Valley, the tourist center of the scenic park visited by millions of people every year.
The National Park Service (NPS) has written to some 2900 people who booked stays in the Boystown area tent lodgings between June 10 and August 24, alerting them to keep an eye out for symptoms of HPS.
The Centers for Disease Control and Prevention (CDC) estimated the number of people who actually stayed in the tent cabins - those who booked plus their guests - at 10 000.
"On August 24, 2012, the tents were disinfected and visitors were relocated. People who stayed in the tents between June 10 and August 24 may be at risk of developing HPS in the next six weeks," it said.
The incubation period for HPS is typically two to four weeks after exposure, with a range of a few days up to six weeks. Symptoms include fever, chills, myalgias, cough, headaches and gastrointestinal ailments.
"The disease often progresses rapidly to respiratory distress, requiring supplemental oxygen and/or intubation, non-cardiogenic pulmonary edema and shock," the CDC said.
"There is no specific treatment available, but early recognition and administration of supportive care greatly increase the chance of survival."
Since the disease was first identified in 1993, there have been some 60 cases in California and 587 cases nationwide in the United States, around a third of which have been fatal.