Public health officials plan to interview and collect blood samples from up to 2,500 Yosemite National Park workers as they hunt for clues in the biggest outbreak of the deadly hantavirus in nearly two decades, a state health official said on Monday.

The voluntary employee screening, scheduled for Tuesday and Wednesday, is the most recent effort to shed light on the rare, mouse-borne lung disease, which infected nine park visitors and killed three last summer.

"This is a highly unusual situation," Barbara Materna, chief of the California Department of Public Health's occupational health branch, told Reuters.

"It is the largest outbreak of hantavirus that we've seen. We're looking at it as an opportunity to learn more about this condition, how exposure happens and how to prevent it," she added.

The U.S. Centers for Disease Control and Prevention sounded a worldwide alert about the virus over the summer, saying visitors to the popular insulated Curry Village tent cabins between June and August were at risk of contracting the disease.

The nine confirmed infections marked the biggest cluster of cases since the disease was first identified in the United States in 1993, when it infected 18 people in the U.S. Southwest.

All but one of the nine infected visitors stayed in Curry Village in double-walled, insulated tent cabins later found to be infested with deer mice. The tiny, white-bellied mice carry the airborne virus in their droppings, urine and saliva.

Among the lingering questions over the outbreak is why hantavirus infected park visitors while sparing employees. Questions will cover living conditions, contact with rodents and hantavirus-prevention training.

Materna and a team of public health officials arrived in the popular national park on Monday to administer the 50-question survey and blood tests examining willing workers' exposure to hantavirus following a smaller pilot study last month. By Monday afternoon, 300 employees had signed up.

The epidemiological study is part of a broader scientific effort, including the first whole-genome sequencing for the hantavirus strain that struck Yosemite last summer.

Humans have never been known to transmit the virus, which kills more than a third of those infected. People can inhale hantavirus when mice droppings mix with dust, especially in confined, poorly ventilated spaces.