I smoked cigarettes for 10 years when I was young. I also experimented with drugs and alcohol, since I came of age during the late 60s and early 70s. Eventually I stopped the bad habits and got into running. After some time I became a national class runner for a few years. I trained so hard at times that I couldn't sleep at night from the cortisol left over from my workout that day.

An additional possibility in my case is that I had a lot of trauma as a child. I've read that childhood trauma is associated with the later development of autoimmune disease. It makes sense to me. Trauma can rewire a child's brain, and cause the nervous system to become hypervigilant. If the nervous system is overly active, it seems reasonable that the immune system is also overly active.