FRESNO, Calif. (AP) — Health officials are set to draw blood from hundreds of Yosemite National Park employees as part of a research project that aims to help scientists better understand a potentially deadly virus carried by deer mice that killed three park visitors and sickened six others this summer.

More than 300 year-round employees will provide blood and answer an extensive questionnaire, as park epidemiologists and doctors with the California Department of Public Health try to determine how many workers might have been exposed to hantavirus pulmonary syndrome over the years, and why some people get sick and others don’t.