A possibility I wonder about is if something in the microbiome could be producing a molecule that gets into the blood and causes havoc. Some post-infectious change in the balance of the microbiome might favor an organism that normally produces an innocuous amount of this molecule. Subsequent expansion of this organism might lead to the production of enough of this chemical to become toxic (if it can get into the blood).
This is pretty similar to the hypothesis which was behind treating supposed candida albicans overgrowth in the GI tract in the 1980's. Diet was thought to be the promoter of candida overgrowth in the gut. Candida albicans is known to produce is acethyaldyhyde - which, when produced by the
liver following alcohol consumption, contributes to hangover symptoms.
Unlike with alcohol consumption, however, under this hypothesis acethyaldyhyde produced by yeast in the gut would
constantly be entering the blood stream.
There's another type of yeast (
Saccharomyces cerevisiae) which can directly produce
ethanol itself in the gut, resulting in
auto-brewery syndrome.
I'm not singling out yeast here. There could be other organisms (i.e. bacteria) in the gut capable of producing toxic molecules that can cross the gut barrier. There may be no problem so long as the number of these organisms remains below a certain threshold.
[As always, pure speculation.]