It always amaze me how much the ICC criteria are resisted here. Maybe because then some would fall out of the grid? is it fear?
Diseases are often not strictly seperated from normal conditions. Very little decays in the teeth may be repaired by the body, mutations of cells that eventually can result in cancer take place normally. On the other hand there might be diseases like Huntington or maybe MS which show abnormality already in their begin.
It would be easily possible to interpretate our disease as an exaggerated mechanism that rather takes place normally. Why are symptoms non-specific? Because it would be a normal mechanism, only that it would have become laughable high (or low) driven. This does not say that there couldn´t built up new feelings (for sure such feelings occure).
So, in this case it wouldn´t be a surprise that there is an overlap to other diseases and to normal bad feelings. It also would easily explain why there is not only a sudden onset (mostly after viral infections) but also a gradual one (maybe e.g. after vaccination?).
I personally think that it´s a feeling-ill mechanism, and for certain, I think, there must be such a mechanism (though it has not been identified). Such a mechanism would explain the non-specificity and the wide range of symptoms, the different onsets from physiological impacts, and possibly an influence on physiological things (assuming a feedback regulation).