I was thinking about how many of us report that we don't seem to suffer from colds or other viral infections since developing ME/CFS. Maybe it's an alternate mode for our viral defences. I have no idea of the timeline of our t-cells' evolutionary development, but maybe our distant ancestors faced a challenge that the modern t-cell response wasn't effective on, or maybe it was the normal mode then, replaced by 'newer, better'. When triggered, the t-cells produced a signal which switched mitochondria into this lower function mode, which in turn made us suffer less from the viral attack. I don't know if we're less vulnerable to viruses, kill them quicker, or just suffer lesser symptoms, but however it works, we seem to benefit from ME/CFS in that way. Given the appropriate viral environment, it could have been enough for evolutionary selection. However, maybe we've lost the 'off switch' somewhere along the way. The mechanism for the off switch might have been blocked by later genetic changes, and without the original threat, there would be no selection against the loss of the switch. I don't have any suggestions leading from this hypothesis, but maybe it will trigger speculation in someone else.