Humans love to find patterns in the world. Sometimes patterns exist, sometimes they are imaginary. Sometimes you can see a pattern that may be interesting and ignore its significance. As a resident I used to say that anyone who smokes three packs of cigarettes a day has to be schizophrenic, it was meant more as a joke, when, in fact, it was later discovered that tobacco helps ameliorate the symptoms of schizophrenia. I need to pay more attention.
Part of my job is to look for patterns as a key to the patients diagnosis. Diseases and pathogens tend to (more or less) cause reproducible signs and symptoms and looking for that pattern is often the most helpful clue towards finding the diagnosis. Of course things are never as easy as one would like, as you have to consider whether you are seeing common manifestations of a common disease, uncommon manifestations of a common disease, common manifestations of a uncommon disease and, the hardest, uncommon manifestations of an uncommon disease. When I have a complex or uncertain cause, I explicitly run through that, and other, litanies so I do not miss a unusual diagnosis.
Chronic Fatigue Syndrome (CFS) has, at least to my way of thinking, two patterns. I see the occasional CFS patient in clinic and, I hope, pay attention to their disease patterns. I keep in mind I may be seeing a pattern that does not exist, but looking for disease patterns is what doctors are trained to do.