(CNN) -- More than a week after announcing two vacationers had contracted hantavirus -- one of them fatally -- Yosemite National Park said Monday, August 27th that authorities have determined another person died after contracting the disease while visiting the park.
In addition to this fatality, Yosemite said in a news release that officials had identified "a probable fourth case of hantavirus."
All four cases involve people who stayed at the park's popular Curry Village in mid-June. The park said its officials are trying to reach those who vacationed in these "tent cabins" from June through this month, when the accommodations were thoroughly cleaned, and urge them to get medical help if they show any symptoms of hantavirus.
Hantavirus cardiopulmonary syndrome is a rare but often fatal lung disease that kills about a third of those who get infected. Don Neubacher, Yosemite's superintendent, noted that people typically don't fall ill with hantavirus until between one and six weeks after they are exposed.
"The health of our visitors is our paramount concern, and we are making every effort to notify and inform our visitors of any potential illness," Neubacher said.
In the United States, the carriers of hantavirus are deer mice, cotton rats, rice rats and white-footed mice, according to the U.S. Centers for Disease Control and Prevention. The virus can be present in the rodents' urine, droppings and saliva, and is spread to people when they breathe in air contaminated with the virus, the CDC says.
The virus is not communicable from person to person.
Earlier this month, the California Department of Public Health said two state residents who came down with the disease may have been exposed to mice droppings or urine that contained hantavirus while staying at the park.
An unidentified 37-year-old man from the San Francisco Bay area died in late July, said Dr. Vicki Kramer, chief of the department's vector-borne disease section.
A Southern California woman in her 40s survived and is recovering, Kramer told CNN.
The two stayed in separate locations at the village, which contains about 400 canvas tent and wooden cabins. Yosemite ranger and spokesman Scott Gediman described the cabins as "very sparse but comfortable."
Park officials have not given details about the one new confirmed case or the "probable" one.
Officials have focused on deer mice, common in the high-elevation eastern Sierra Nevada region. The mice are gray or brown on top, with white bellies. Their ears have no fur.
"Rodents can infest a whole range of these structures," Kramer said. "Deer mice can get in a hole one-quarter inch in diameter."
In addition to cleaning the 400 camp structures, park officials have said they've increased routine measures to reduce the hantavirus risk.
"You cannot eliminate all the mice," Kramer said. "There are a lot of people and snacks that people bring into their tents or cabins."
Prior to this year, Yosemite National Park saw one hantavirus case each in 2000 and 2010.
There is no specific treatment for a hantavirus infection, according to the CDC, but the earlier a patient is brought to intensive care, the better.
Diagnosis is difficult, because early symptoms of fever, muscle aches and fatigue often are confused with the flu.
"However, if the individual is experiencing fever and fatigue, and has a history of potential rural rodent exposure, together with shortness of breath, (that) would be strongly suggestive of HPS," the CDC says.