Public health officials have begun examining blood samples and questionnaire results from dozens of Yosemite National Park workers who volunteered for a study designed to shed light on an unprecedented outbreak of deadly hantavirus.

Reuters reported that this week’s screening, which involved 96 workers, is the latest effort by officials to uncover clues about the rare, mouse-borne lung disease that has infected at least nine park visitors and killed three since June.

“This is a unique opportunity to learn more about potential exposure to hantavirus from the people who lived and worked in close proximity to this cluster of cases,” park spokesman John Quinley said Thursday.

One aim is to help researchers design a study of the California park’s more than 2,500 workers in a search for clues about how the virus infects people and how to prevent it.

Among the lingering questions over the outbreak is why hantavirus infected park visitors while sparing employees.

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.

The park closed the infested tent cabins at the end of August, after learning of two of the hantavirus deaths.

This week’s screening will examine park workers’ blood for antibodies signaling exposure to the hantavirus. Employees also answered questions about where they live, their contact with mice, their job duties and their hantavirus training, Quinley said.

“We’ve had a lot of employees go through hantavirus training,” he said. “We’re interested in knowing if the training works.”

The voluntary study is part of a broader scientific effort that will include the first whole-genome sequencing for the hantavirus strain t h at struck Yosemite over the summer. It marked the biggest cluster of cases since the disease was first identified in the United States in 1993.

At that time, when hantavirus pulmonary syndrome infected healthy young adults in the Four Corners region of Utah, Colorado, Arizona and New Mexico, Navajo elders linked the outbreak to an abundance of pine nuts and an explosion of mice coming out for the feast.

Since then, more than 600 Americans have been diagnosed with hantavirus pulmonary syndrome. The incurable disease kills more than a third of those infected.

To date, humans have never been known to transmit the virus. People can inhale the hantavirus when mice droppings mix with dust, especially in confined, poorly ventilated spaces.