Yosemite National Park officials plan to offer testing to all employees in the park to determine whether they’ve been infected with a deadly mouse-borne virus.

Park spokesman John Quinley said Thursday the voluntary testing will be available to all employees of the National Park Service and the park’s concessionaire, DNC Parks and Resort. He declined to say when the testing would start.

The California Department of Public Health conducted a pilot testing program Wednesday, taking blood samples and questionnaires from 96 National Park Service employees.

There have been no confirmed or suspected hantavirus cases among employees so far. But nine people who visited the park this summer have been infected, the majority after staying at the “Signature” cabins in Curry Village. Three of them have died.