In Japan, Chronic-Fatigue-Syndrome is called low natural killer cell disease. The objective criteria for diagnosing it is a patient having low natural killer cell activity, and all the more serious immune system problems that could cause that are ruled out(as well as ruling out a vitamin D deficiency, or deficiency of vitamin D cofactors). I guess if one needs to prove they have Chronic-Fatigue-Syndrome to get disability benefits, a good test to do is natural killer cell activity.
The link below states that they believe that some of the causes of Chronic-Fatigue-Syndrome are heavy metal toxicity(especially mercury) or pesticide toxicity.
Vitamin D deficiency may also cause low natural killer cell activity. Vitamin D deficiency may also block heavy metal detoxification, as vitamin D plays a role in boosting glutathione levels. Low natural killer cell activity can set the stage for candida overgrowth.