Ashton Kutcher says he is "lucky to be alive" after a rare disease affected his ability to see, hear and walk.

The 44-year-old actor detailed the symptoms he suffered in an upcoming episode of Running Wild with Bear Grylls: The Challenge.

In a clip for the episode obtained by Access Hollywood, Kutcher said: "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. It took me like a year to build it all back up."

"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.’"

"I'm lucky to be alive," he added.

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The Two and a Half Men star also shared how his autoimmune disorder gave him a change in perspective, as he expressed how life got "fun" for him when he stopped hiding from his "problems".

"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?" he said. "You start surfing on top of your problems instead of living underneath them."

Kutcher shares two children with his actress wife Mila Kunis, seven-year-old daughter, Wyatt, and five-year-old son, Dimitri.