I have a theory, pure speculation of course, that relates to the age part of your list. I think that in our late 60's/early 70's the immune system declines and something similar to menopause happens . Cortisol levels fluctuate, much like hormones do during menopause. The system is fading but struggling to stay going at the same time. The levels of cortisol or some other body chemicals being out of whack is what causes the autoimmune response. It could also be that we exercisers have stronger immune systems that cause more havoc when disrupted.