I have been thinking about this and remembered that even as a very healthy child I could not run, swim, bike as far as other kids my age--I would just poop out. This is making me wonder if there is a genetic component that affects the production of aerobic energy (or glycolysis) that predisposes us to later develop ME/CFS. (@Rose49 ) Looking back, there was never I time when I could run any distance, jog, take an aerobics class etc. Of course this was greatly exacerbated when I developed ME/CFS. Do any others have this history?