I wonder if it truly is something that occurs with
any viral infection or something that
can occur in some viral infections in some or all people. Could it represent a brief period of latent viral reactivation, that causes symptoms that are limited in time and scope (a very light and transient form of ME)? The difference being that in a postviral fatigue syndrome of, say a few weeks or months, it persists for a bit longer but does spontaneously resolve; but for ME/CFS that resolution does not occur and the latent viral reactivation persists and is in fact reinforced. In the extreme situation, the reinforcement is much more marked, the situation rapidly worsens, symptoms are very much worse and you have "severe ME/CFS".
I've had in my mind the concept of an "arrow of recovery" which can be pointing sharply or shallowly upward, flat, or shallow or sharply downward. This would depend on physiological factors, possibly including things like genetics, or maybe factors relating to the specifics of viral latency. For those whose arrow is pointing upward, they would recover unless they do something unwise. For those whose arrow is pointing very high, they have more license to exert and push-through - and it may be that in those, the benefits of exercise are gained and outweigh any penalty; and recovery could then be accelerated. Eg in the Garner situation, he may have been inevitably recovering, due to a favourable set of physiological circumstances, although that's not how he saw it as we've subsequently heard ad nauseam.
For long-term pwME, perhaps the arrow is flat for many years, but something else happens (eg a new viral infection) and tips the arrow downward and symptoms worsen.
When I initially read your comment, which used the word "included", I wondered if we ought to draw a distinction between fatigue and other things like nausea and OI, ie as separate symptoms. But I think the way you phrased it could be right. Ie if fatigue (in the sense of exhaustion and reduced energy, as opposed to the feeling of being tired and wanting to sleep as per most media) underlies these other symptoms. A hypothetical explanation would be: OI being caused by inadequate ATP supply to endothelial cells leading to endothelial dysfunction; nausea due to inadequate ATP in gut mucosal, smooth muscle and nerve cells with resulting poor and uncoordinated motility and increased gut wall permeability and inflammation.
Or it could be that fatigue and PEM are actually separate but related effects, where PEM relates to the sort of biochemical changes (or lack thereof) that Maureen Hanson's team are uncovering. Ie does lack of energy production capability underlie the inability to prevent PEM? Or is it more related to changes in the immune system such that what should be done to clear up the toxic effects of exertion can no longer be? Either way I would imagine dysfunctional* mitochondria could be at the centre.
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* Dysfunctional might not be the right term, see
Multifaceted mitochondria: moving mitochondrial science beyond function and dysfunction (2023, Nature Metabolism)