The hantavirus warning has been extended to 230,000 people after three people died from the rodent-borne disease said California's Yosemite National Park.
"We are reaching out to additional overnight visitors to raise awareness about this rare disease and to ensure they know where to find information regarding hantavirus," the park said in a statement on its website Thursday.
"This public service message is intended to reach more than 230,000 overnight guests who stayed in the park since early June."
The park said a total of nine visitors have come down with the virus since June, with six having recovered.
Symptoms of the disease, which can develop within two to six weeks, include fever, chills, cough, headaches and gastrointestinal ailments, but can rapidly escalate to breathing difficulties and death.
Yosemite had already notified 10,000 people worldwide who had stayed overnight in the "Signature Tent Cabins" in an area known as Curry Village, where the virus was first detected, between June 10 and late August.
Some 75,000 tourists a day visit Yosemite, with the majority flocking to Yosemite Valley, which includes Curry Village.
A 2008 study by California's Public Health Department said the virus can be found in about one in five of the deer mice in the state's forest service facilities.
The animals spread the disease through urine, feces and saliva. It is often contracted by inhaling contaminated dust.
Since the disease was identified in 1993, there have been 60 cases in California and 587 nationwide.
More information about the outbreak can be found at www.nps.gov/yose/planyourvisit/hantafaq.htm