"He was filming a series on haunted houses where he had been lowered into a lake where a body was buried. 'I think it's because the water was so cold', he told me.
"The next morning he rang: 'Mum, I can't see out of one of my eyes, and my head is still killing me'. 'You've got to see a doctor. Just get on a plane and come home'.
"I knew straight away it wasn't good. Jack had also complained previously about a condition he had where his legs got really hot and it felt like he had pins and needles.
"The symptoms sounded suspiciously familiar. In 1992 I'd done some research into multiple sclerosis when I thought Ozzy might have it. Jack went to see an eye specialist who said it was neurological. He went to the neurologist who said he needed an MRI scan.
"I went with him. I was watching the images of Jack's brain and the technician pointed to a couple of white spots. 'Looks like he may have had a stroke', he said.
"It scared the sh*t out of me. Soon after Jack was admitted to Cedars-Sinai hospital in LA for tests. He was very on edge, not just about the results, but because he had a baby and a gorgeous wife Lisa.
"And all the time he was thinking, 'Oh God, am I even going to be able to walk for much longer?'
"If anything happens to you, you cope with it. But when it's one of your children, you just can't get your head around it. When Jack was diagnosed, he was angry. It was, 'Why me?' After a couple of days the tears came.
"Since that diagnosis in June last year Jack's eye is a lot better. He has about 75 per cent vision and can see certain colours.
"But that's probably as good as it's going to get. MS leaves its scars, that's what the name means. The main problem he gets is numbness and tingling in his legs, though I will never know how much because he rarely complains. He's a fighter."
Sharon also revealed that Jack had undergone revolutionary stem-cell treatment
"Jack's results were excellent," Sharon write. "Did the therapy work? It certainly doesn't do any harm. Jack has dropped a load of weight and is feeling good, so fingers crossed.
"It's not a cure, but if it gets his immune system in top order, it might stall an outbreak or even stop it from happening. For now, that's all we can hope for."
• Unbreakable by Sharon Osbourne is to be published by Little, Brown on October 10 at £20. © 2013 Sharon Osbourne