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The cause is not known at this time. There does seem to be some link to immune & gut microbes though. There's also been links to brain matter size, past illnesses & other stresses.
I got CFS after I quit nursing my daughter. Wasn't too bad till after I had 2 major surgeries & 2 minor surgeries all within 4 months. I don't know if was all the hours of anesthesia or the stress or what but my health completely deteriorated after surgeries & continues to do so every year.