Myalgic Encephalomyelitis is a chronic illness where the patient's immune system attacks their mitochondria (of the powerhouse of the cell fame). As a result the patient has less energy to do stuff. And if they overdo it (which they will only know afterwards) they get really sick with flu like symptoms and pain, and their threshold gets lower.
The only known treatment is pacing, ie making a lot of pauses. In bad cases the patient can't do anything and can only lay in bed with earmuffs and blindfold to block out noise and light.
Oh how I wish