Trauma, or shock, eventually leading to a condition I often think of as "Post-Traumatic Stress of the Immune System. Perhaps secondarily (or not), it also almost always leads to a degree of Dysautonomia, or dysfunction of the Autonomic Nervous System, leading to many (to most) of the symptoms associated with ME/CFS. A delicate balance is upset, and the body struggles to regain its homeostasis, but is unable to for various reasons. This inability is often from ongoing stress such as nutritional deficiencies, chronic toxic exposures, genetic factors, EMFs, etc.
I've been fairly amazed over the years by the wide variety of triggers that can lead to ME/CFS. Most of us are familiar with shocks to the immune system like viruses, bacterial infections, mold exposures, parasites (protozoa), etc. But there are many other kinds of shocks that have been reported, like chemical exposures, vaccinations, emotional shocks, accidents, dental/oral infections, and other physical shocks, such as the young policeman who dove into a cold lake to save a drowning victim. Once the stress settles in, it seems to take residence in our bodies, and then settle in deeper and deeper over time. UNLESS, we're fortunate enough to recognize the stress factors that are unique to each of us, and find corrective measures--often unique to us as well
Many more paragraphs of my thoughts left unsaid!