As noted before, I also believe what we call CFS is not one disease, but several very different diseases that share many symptoms, most prominently a very pronounced feeling of fatigue and exhaustion. I believe there is a subgroup of CFS patiens in which the disease is caused by semi- or nonpermissive herpes virus infection (esp. EBV) throughout the body (Dr Lerner's theory).
In this group (to which I think I belong), I believe there is a chronic state of nonpermissive or semi-permissive herpes virus infection that is taking over the body step by step, especially the heart, the brain and the kidney. Infected cells are dysfunctional, cell metabolism is compromised and cells are dying off via apoptosis. For some reason, patients' immune system cannot control or clear the infection, and it is spreading slowly throughout the body, thus creating ever greater dysfunction in infected organs and infecting more organs and tissues, e.g. increasing muscle pain over time.
Once the infection has become more widespread throughout the body and the longer it lingers, the worse the symptoms get. Since the function of critical organs, esp. the heart, and metabolic function of the organism as a whole is compromised, increasing metabolic stress through excercise makes the problem worse. I also suspect that metabolic stress like exercise somehow helps the virus spread, e.g. by increasing the rate of apoptosis and thus releasing viral particles into the blood and nearby tissues. That's why patients get worse when they excercise, up to a point in which the system is in such a bad shape that hardly any metabolic stress can be tolerated.
The good news: Antivirals seem to help (at least to some extent and very slowly).