Los Angeles, California - Ashton Kutcher opened up on his private health battle with a rare autoimmune disorder that affected him badly.

On Monday, Access Hollywood shared a preview from the star's appearance on National Geographic’s Running Wild with Bear Grylls: The Challenge.

In the clip, Kutcher revealed his recent battle with the rare autoimmune disease vasculitis, which causes inflammation of the blood vessels, restricting blood flow and causing organ and tissue damage.

"Like two years ago, I had this weird, super rare form of vasculitis, that like knocked out my vision, it knocked out my hearing, it knocked out like all my equilibrium," the 44-year-old actor shared with the show's host.

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"It took me like a year to build it all back up."

Kutcher noted that he's "lucky to be alive" after overcoming the scary battle.

"You don't really appreciate it, until it's gone. Until you go, 'I don't know if I'm ever going to be able to see again, I don't know if I'm ever going to be able to hear again, I don't know if I'm ever going to be able to walk again.'"

Kutcher's latest update is all the more remarkable because he's known for keeping his personal life as private as possible. But his difficult experiences have given him a brand new outlook on life.

"The minute you start seeing your obstacles as things that are made for you, to give you what you need, then life starts to get fun, right? You start surfing on top of your problems instead of living underneath them," he said.