Yosemite officials told 1,700 past visitors on Tuesday they may have been exposed to a rodent-borne disease already blamed for the deaths of two people who stayed at the park.
The email alerts involved hantavirus pulmonary syndrome, which can be carried in the urine, saliva and feces of infected deer mice.
All of the at-risk visitors had stayed in the "Signature Tent Cabins" in Yosemite National Park's Curry Village.
Yosemite officials warned those who stayed there from mid-June through the end of August to beware of any symptoms of hantavirus, which can include fever, aches, dizziness and chills.
Park officials told people to seek medical help immediately for such symptoms. There is no specific treatment for the respiratory illness.
Two other people were infected and were expected to survive.
Federal epidemiologists learned over the weekend of the second fatality.
Earlier this month a man from Alameda County in the San Francisco Bay area died, and a woman from Southern California was sickened after staying in infected tent cabins in Curry Village, a family friendly area with the park's lowest-cost accommodations.
The four people known so far to have contracted the illness stayed around the same time in June. Federal health officials say symptoms can develop up to six weeks after exposure.
Of the 587 documented U.S. cases since the virus was identified in 1993, about one-third proved fatal.
Thousands of people visit the park every month, so it would be impossible to track everyone who had set foot in Curry Village, officials said.
Curry Village is located at the base of the 3,000-foot promontory Glacier Point.
Park spokesman Scott Gediman said the Delaware North Co., which runs the park's lodging facilities, is working to shore up cabins to protect park-goers.
"There are rodents and some are infected and that's what happens," Gediman said. "This is a wilderness setting. It has nothing to do with the cleanliness of the cabins."
This year's deaths mark the first such fatalities in park visitors, although two others were stricken in a more remote area in 2000 and 2010, officials said.