(CNN) -- Around 1,700 people who visited Yosemite National Park from mid-June to the end of August are being advised to seek medical attention if they exhibit hantavirus symptoms.
The park sent letters to the visitors Monday and Tuesday, explaining that hantavirus cases had been uncovered and that visitors should see their doctors if symptoms surface, park spokesman Scott Gediman said. In recent months, two park visitors have died from the virus.
"Early medical attention greatly increases the chance of survival in cases of HPS," or hantavirus pulmonary syndrome, the letter says. "It is recommended that if you, or anyone in your party, has any of the symptoms listed ... particularly fever, muscle aches, or cough, that you seek health care immediately and advise your health care professional of the recent cases of HPS."
"An outreach effort is currently underway by the park concessioner to contact visitors who stayed in 'Signature Tent Cabins' at Curry Village," the park said in a statement.
Hantavirus pulmonary syndrome is a rare lung disease that kills about a third of those who get infected. Symptoms can include fever, muscle aches and fatigue, though it is not communicable from person to person.
"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 U.S. Centers for Disease Control and Prevention said.
In the United States, the carriers of hantavirus are deer mice, cotton rats, rice rats and white-footed mice.
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.
On Monday, Yosemite announced that a second person had died after contracting the disease while visiting the park. In addition to that fatality, the park said in a news release that officials 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."
Don Neubacher, Yosemite's superintendent, said 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.
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. Gediman, the park spokesman, 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."
Before 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.