MY 17 years with ME/CFS has been a progression through different groups of symptoms. The first 10 years was more Immune type symptoms with the "never ending flu", sore throat, swollen lymph glands, severe fatigue, cognitive dysfunction, muscle pain and sleep problems.....but I didn't have PEM. Then 7 years ago the disease evolved into being Neuro. The Immune type symptoms stopped completely, but now I began the really bad ones.....Severe OI with NMH and POTS, Severe cognitive problems, and PEM.....currently PEM is by far my most severe and disabling symptom. So for me, the PEM came on with the progression of the disease. Regardless, it obviously is a "cardinal symptom" and should be a serious research focus for pinning down a biological marker and leads to etiology. I really appreciate these posts. If we could find a way to limit the PEM alone, many of us would be so much higher functioning.