Yosemite officials are scrambling to understand what caused the hantavirus  that has killed two people and sickened at least one other in an outbreak of the rare rodent-borne disease described as "unprecedented."

After announcing Monday that a third case of the disease had been confirmed — resulting in the death of a Pennsylvania man — and a fourth probable, Yosemite officials emailed about 1,700 people who stayed in the park's popular Curry Village, asking them to seek immediate medical attention if they showed the flu-like symptoms of the disease.

Last week, park officials revealed that a 37-year-old Bay Area man died and an Inland Empire woman in her 40s had been sickened after contracting hantavirus in the park. All four people believed to be affected by the disease stayed in Curry Village's "signature tent cabins" in mid-June.

Dr. Barbara Knust, an epidemiologist with the Centers for Disease Control and Prevention, said it was “very rare” to have more than one reported case of hantavirus in the same location in a given year

Transmitted through the urine, droppings or saliva of infected rodents, hantavirus pulmonary syndrome takes between one and six weeks to present in humans, officials said. The symptoms — fatigue, fever, muscle aches, nausea, vomiting and abdominal pain — are often confused with the flu, Knust said, but can quickly worsen as one’s lungs begin to fill with fluid, causing respiratory problems that can be fatal.