FRESNO, Calif. (AP) — Yosemite National Park will offer testing to all employees at the site to determine if they have ever been infected with a deadly mouse-borne virus, officials said Thursday.

The testing to be done by state health officials will be voluntary and available to all workers for the National Park Service and its concessionaire, DNC Parks and Resort, park spokesman John Quinley said. He declined to say when the testing would start.