To make things eve more complicated, it seems that a number of other conditions are frequently connected to or overlapping with Chronic Fatigue Syndrome. In such cases Chronic fatigue Syndrome can only be diagnosed after the other conditions have been established and treated or mysteriously resisted the standard treatment procedures.
These include thyroid disorders, anemia, diabetes, fibromyalgia, multiple chemical sensitivity, Gulf War syndrome and post-polio syndrome, irritable bowel syndrome and temporomandibular joint pain and chronic pelvic pain syndrome.

So this is quite a number of conditions to be either diagnostically excluded or treated in the first place.

There also exists an overlapping with severe depression. But until now it could not be established whether depression was an underlying cause for Chronic Fatigue Syndrome or just a consequence of suffering from a chronic illness. As a matter of fact people suffering from any chronic illness will show a much higher tendency to develop a severe depression than healthy persons.