Jack Osbourne admits being diagnosed with Multiple Sclerosis (MS) was the "scariest moment" of his life.

In an open letter published by People on World MS Day on Wednesday, Jack - who has daughters Pearl, four, and Andy, 11 months, with wife Lisa - wrote: "Multiple Sclerosis. Two words that changed my life forever.

"When I learned I had relapsing-remitting multiple sclerosis four years ago, it was the scariest moment of my life. I was only 26 years old, I had just become a father and I thought my whole life was in front of me - but there I was, being told I had an incurable and often debilitating illness. I had so many emotions and so many questions, and, frankly, I was terrified of the unknown."

Jack will be "forever grateful" for the support his family have always given him as he deals with his condition, especially in the early days of his diagnosis.

He continued: "After I was diagnosed, I turned to my family first, and then to the Internet. I tried to gather as much information as I possibly could on what MS is, how it manifests and what that would mean for me.

"I was overwhelmed, and it was a lot to process and digest.

"My family was an incredible support system for me, helping me understand and wrap my head around the disease, and to them I am forever grateful."

And the 30-year-old star credits his family for giving him the drive to fight against the disease.

He wrote: "In my eyes, World MS Day is just as much about our family and friends as it is about everyone living with MS. It's not just individuals who can suffer.

"My wife and kids are my reason for wanting to endure whatever this disease throws at me.

"They are with me through thick and thin, and vice versa.

"So many caregivers and family members make such a selfless sacrifice in order to help make a life possible for many people living with MS. They are the heroes to me, in this battle.

"So to all my family and friends who had supported me and been there for me, I'd like to say thank you, I love you. This day is for you as well."

While Jack isn't "thankful" for the condition, he does think his diagnosis has helped him change for the better.

He explained: "I will never say that I am thankful for MS, but I will say that without MS, I don't know if I would have made the necessary changes in my life that have changed me for the better. I believe that one day we will all celebrate a cure together, and what a day that will be."