It leads to worsening of the illness. A CFS patient who goes beyond the boundaries of his energy reservers shrinks these. That leads to smaller and smaller energy reservers, till you can't get out of the bed, sit upright, read longer text, type longer comments.

It aggravates the symptoms of CFS (fatigue, pain, headaches, etc). It can last multiple days after exercise. (Source: I know a person who suffers from CFS)

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.

A quick google of "CFS exercise" lead me to few articles, this is the one I'm reading now: https://www.npr.org/sections/health-shots/2017/10/02/5543693...

The information is sourced from the official CDC guidelines on ME/CFS: https://www.cdc.gov/me-cfs/index.html