In a sneak peek at this week's episode, the "That '70s Show" star opens up about a private health battle when asked where his strength comes from.
he revealed in the footage, first shared by Access Hollywood. "It took me like a year to build it all back up.
According to the Mayo Clinic, vasculitis is an inflammation of blood vessels which can lead to organ and tissue damage.
Treatment includes taking a corticosteroid like prednisone or methotrexate to get the inflammation under control.
"The minute you start seeing your obstacles as things that are made for you, to give you what you need, then life's supposed to get fun,
Hearing Ashton's story about his autoimmune disease, that's a terrifying journey but it's also part of why he's strong and resilient," said Grylls after Kutcher's reveal.
Ashton's episode of "Running Wild with Bear Grylls: The Challenge" airs Monday on National Geographic.