Physician here. Imo CFS is best understood as the endpoint of an evolution within the nervous system that is increasingly called "central sensitization syndrome." (CSS) This syndrome is caused by a chronic dissociative or avoidant stress response, usually having it's origin in early childhood adversity. Prior to developing CFS, most individuals have other manifestations of CSS, such as IBS, chronic pain, TMD, etc. Ev…
I had high energy levels until I suddenly got sick, and had a mild fever for about a day. After that I had the sort of fatigue one feels when having the flu, except it lasted for about 8 months, and then gradually lifted. After that my energy levels totally recovered and stayed normal for over a decade. All tests, including psychiatric evaluation were normal so it was considered CFS.