I had CFS that originated as a post viral illness after Epstein Barr Virus. The kicker is that I was in the military when this happened. By Tuesday every week, after 2 PT sessions, I was on a downhill slope. If I did a third session on Wednesday it was likely I was spending 5-7 days in bed. I'd been hospitalised for exhaustion.

I had every medical test under the sun and everything came back negative except my recent markers for EPV. I was lucky, my OC signed off for me to not to exercise and I didn't do PT for 9 months. During this period my CFS very slowly got better and I appear to be mostly better now.

You don't want to get CFS while in Army

not only days, it can be forever.