- I think direct post-viral fatigue, including post-COVID is in the head. Specifically, in brain tissues. Something along the lines of an encephalitis or MS -- encephalopathy is probably the best term. A persistent inflammatory condition related to immune dysfunction or persistent infection by something (like reactivated latent viruses, COVID, etc).
- It's very hard to separate post-viral neuronal dysfunction from anxiety, depression, ongoing dementing processes, coincident head injuries, coincident brain disorders, sleep disorders and the like. It's all in the brain after all. (These aren’t exclusive conditions, so some unlucky person must get all of them at once. Heck, for all we know depression is partly a postviral damage disorder.) We need better tech -- maybe a combination of anatomic and functional brain imaging will help one day. Maybe it will be something we can diagnoses between MRI and lumbar puncture/CSF samples.
- I think one day we'll find post-COVID encephalopathy (PCE I'll call it) occurs in less than 1 in 500 ever-infected people and in most it improves over 3m to 1y. In most, but we now believe MS is an infrequent or rare sequelae of Epstein-Barr infection. So we gotta worry that some PCE is not going to get better unless we come up with new treatments.
- There are almost certainly other viruses that cause similar conditions (post-viral encephalopathy). Maybe non-COVID coronavirus URIs aren't as benign as we thought.
Maybe in 2030 I'll come across this and update with how it turned out.