So, lack of knowledge is weakness, or vulnerability?
Or being alone in the dark with the compass spinning?
How to handle ME / CFS (and there's enough just in those preceding characters to let the mists of uncertainty start rolling in, if not the fog of war) is going to depend on what it is, where and what the underlying cause or causes.
Without that one is stumbling in the dark metaphorically as well as literally, and facing a very big extra headache. Just when one doesn't need it. Like a Balrog, really.
"A Balrog," muttered Gandalf. "Now I understand. What an evil fortune! And I am already weary."
Another classic "I wouldn't start from here". Far better to attack the question of CFS when one doesn't have it. But there it is.
If gathered in the drawing room of the country house, who will be there?
Thyroid issues (T3, T4),
and these are somewhat shifty in manner, as they are not regularly tested for, as yet, by the NHS.
"All in the mind": considered a popular suspect, by a good number of detectives. The victim did it to himself, all unaware. Now there's a plot twist.
But some sleuths appear to have made up their minds on this in advance of the evidence. Sherlock Holmes might have a word with them about that. However, if guilty, then the steps to be taken to recover the stolen energy and stamina will be very different than if a different culprit is actually guilty, and just needs locking up or a judicial execution.
Gut bacteria: bad ones, or unbalanced ones.
Hydrogen sulphide from the above: the body is continually being gassed.
Other food suspects: toxic reactions and autoimmune ones.
Outsiders seen lurking locally:
Lupus, the Epstein-Barr virus, Lyme disease.
Further, as in a bad novel, the actual guilty party might not even have made an entrance yet, to be suspected.
With a variety of detectives pointing the finger in a range of directions,
we're nothing like at the final chapter, yet.
And yet the poor victim has got to trust somebody and their advice on the right path to pursue*.
The wrong one, of course, quite possibly making the whole thing worse.
Who to trust?
Now that comes back to epistemology.
Where the honest man (correct), the honest man (mistaken) and the conman all have no trouble in saying "Don't worry, you can trust me."
Has anyone got the last chapter?
And can also convince me it's the right, genuine, last chapter?
If not, this wretched thing is still an improvisation, for the forseeable future.
*unless predisposed to just lie there passively.
And suffer the gratuitous insults of local villagers who are certain there was no crime, except fraud.