I believe IamCurious is referring to chronic exercise, which is no doubt degenerative to our organs. The kind I am referring to is short high intensity exercises and sprints, maybe 3-4 times a week. Here's a
primer on it. Studies show these exercises are just as effective as the long sessions that people tend to do (which are overdone most of the time). Science also says walking (as in the sun) is also the premiere form of exercise and offers the most benefits out of anything else.
I also believe there is a psychological component to autoimmunity, and people feel like they need to overcompensate for other areas they "think" they are deficient in. The result of this is chronic exercise, marathons, weight lifting (see aguywithuc's posts) and so fourth. These guys also tend to be work horses. Maybe this has something to do with our gut bacteria influencing the neurotransmitters. Maybe this is just me profiling, but this trend is a cause for concern.
I used to overdo it myself by running 3-4 miles daily, but didn't know the harms of it. Unfortunately, I see many people in fitness centers doing the same thing these days. Chronic cardio like that is really bad for the heart and so is chronic weight lifting for our organs. There are studies on it and people can find it on google.