I have had ME/CFS for 38 years since catching Glandular Fever from my daughter. I have had 2 lots of cancer during that time, plus developed other illnesses and conditions, amongst them Fibromyalgia, EPI and Osteoarthritis, so my health has been very complicated. For years I was bedbound, and although I manage to live on my own nowadays, I still go back to bed in the afternoons.
I believe, like Maybee70 says, that the microbiome is involved and have had mine tested and tried to rectify the imbalances that showed up, but I haven't improved. Glandular Fever and other nasty viruses such as Covid, play a big part in triggering ME.
Sending all my very best wishes to Choughdancer, to your daughter HelterSkelter and to anyone suffering. It truly is a horrible condition and some doctors' attitudes only make it worse for us.