If people have vague symptoms that are uncertain and ambiguous, then their opinions on the nature of their illness are probably not terribly useful or accurate.
But if there exists a severe illness, it seems almost certain that there are some people out there who have a mild form of it that is ambiguous and uncertain because there is a spectrum.
More generally, I believe that evolution implies that diseases evolve in a manner such that people find them hard to identify. Because humans are pretty ruthless at eliminating diseases that catch their attention. HIV is an example of a virus that had gotten very close to coexisting, but then people noticed it and have been trying to wipe it out. Vague chronic problems that are blamed on the victim are inherently what everything logically has to become in the long run.
To take the argument to the extreme, even hypochondria must be a physical disorder, even though of course like cancer etc. there might be many causes that are arguably not the same.