I think you might be looking at a weakened immune system and a possible correlation with stress.
I have found that at times of stress in my life, I am more vulnerable to colds and acne. I RARELY ever saw a pimple on my face, but the occassional ones I got where at times when I was sick.
Acne is a mild infection, and it is more likely to show up if your immune system is stressed (just like other infections).
If your health declines, or you gain alot of weight, that does not directly cause acne, but it stresses your body and immune system, so I would be the least bit surprised if there is a correlation.