It seems cytotoxicity (the ability of a stressor to damage cells) can be a phenomenon that is delayed in comparison to the original stimulus. Ie. it seems depending on the immune or chemical leading to cytotoxicity it can occur instantaneously to a week after. [1] It seems also a well regarded finding that some immune cells can produce cytotoxicity. [2] And it is well known that exercise has substantial effects on the immune system. [3] [4] Could this be related to ME/PEM?