The tourists who died had stayed in Curry Village, a popular camping area tucked below the park's sheer granite walls, a Yosemite spokesman said on Tuesday. A third visitor was sickened by the virus but recovering.

Investigators were looking into whether a fourth visitor was suffering from the illness, which is carried by wild rodents. All four stayed in the area's tent cabins on overlapping days in mid-June, spokesman Scott Gediman said.

"We are encouraging anyone who stayed in Curry Village since June to be aware of the symptoms of hantavirus and seek medical attention at the first sign of illness," the park's superintendent, Don Neubacher, said in a statement.

Symptoms, which include fever, headache and muscle ache, appear one to six weeks after exposure. The virus kills slightly more than one-third of those infected.

Nearly 4 million people visit Yosemite each year, and roughly 70 percent of visitors congregate in the Yosemite Valley, where Curry Village is located.

The cabins have been cleaned and remain open, and officials said they do not consider there is any immediate health risk.

A 37-year-old Northern California man died of the virus at the end of July, and a man from Pennsylvania died of hantavirus two weeks ago, Gediman said.

The victims, who have not been publicly identified, did not know each other, he said.

After determining that those stricken all stayed in the tent cabins during the same week, park officials began reaching out to 1,700 other people who stayed in the area, and told public health officials across the nation to be on the lookout for signs of the virus.

California has reported 60 cases of hantavirus cardiopulmonary syndrome since it was first recognized in 1993. Nationwide, 587 people were sickened with hantavirus between 1993 and 2011, according to government data.

Public health officials believe that only deer mice carry it in California. As many as 20 percent of Yosemite's deer mice carry hantavirus, Gediman estimated.

"This doesn't mean we're going to attempt to eradicate the mice population," he said. "The deer mice are an important part of the ecosystem."

Most hantavirus infections result from breathing air contaminated with rodent urine or droppings, usually in small, confined spaces with poor air circulation. People can also be infected by eating contaminated food, touching contaminated surfaces or being bitten by infected rodents.

Blood tests can diagnose the virus. There is no treatment, but supportive care can improve survival chances.

(Editing by Mary Slosson and Xavier Briand)

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