Ashton Kutcher says he feared autoimmune condition vasculitis would cost him his sight and hearing. Here's what it is and how it impacts the body.

• Actor Ashton Kutcher said he contracted a condition called vasculitis about three years ago.
• The condition is caused by the body attacking its own veins, making it harder for blood to flow.

Actor Ashton Kutcher said he feared he would lose his hearing, vision, and ability to walk because of a condition called vasculitis.

The actor opened up about his vasculitis for the first time on "Running Wild With Bear Grylls: The Challenge," according to a preview shared by Access Hollywood.

The condition "knocked out my vision, it knocked out my hearing, it knocked out like all my equilibrium," Kutcher told Grylls, per the preview.

Though he has fully recovered, the actor said he felt "lucky to be alive" in footage from the show.

Here's what we know about vasculitis.

It is an inflammation of the blood vessels

Vasculitis is a term that means the body's blood vessels are inflamed, per NORD, a rare disease patient association.

That can cause swelling, narrowing, weakening, or scarring of the vessels.

Because the blood isn't able to flow properly, it can cause blood clots, organ failure, and even aneurysms in the most life-threatening cases, per the American College of Rheumatology.

Vasculitis makes the body attack itself

Vasculitis happens when the body's immune system starts attacking its own blood vessels by mistake. There are lots of conditions that can cause vasculitis, and it's not clear which caused Kutcher's.

In some cases, it can be triggered by a bad reaction to some infectious diseases or an allergy to medication. In others, it will be linked to an auto-immune disease, like rheumatoid arthritis.

"All forms of vasculitis are rare and each have their own challenges," the Vasculitis Foundation said, commenting on Kutcher's diagnosis.

Symptoms can include pain, diarrhea, and bloody urine

Symptoms of vasculitis vary wildly depending on which blood vessels are affected. They range from muscle and joint pain, fever, fatigue and weight loss, to ulcers, skin lesions, diarrhea, bloody urine and more, per NORD.

Doctors will rule out more likely causes for these symptoms before asking for diagnostic tests to spot vasculitis, per the American College of Rheumatology.

Vasculitis is often mild, but it can also be severe and life-threatening, per the Vasculitis Foundation. Many will recover on their own and their condition will be short term, but relapses are also common, per the Foundation.

Anti-inflammatory medication can be prescribed to treat the condition.

Kutcher has had a full recovery

In a tweet commenting on the Access Hollywood preview, Kutcher said the flare-up happened three years ago, and he since he has "fully recovered."

He said he will be running the New York Marathon later this year for Thorn, a non-profit organization that he cofounded to fight the sexual exploitation of children.

"All good. Moving on. See you at the 2022 NY Marathon with Thorn," he said in a tweet