The fact that people with Myalgic Encephalomyelitis originally and demeaningly called Chronic Fatigue Syndrome can’t exercise.
It was first believed to be a mental health disorder where people are scared of doing activity. And patients who said exercising made them worse were treated for hysteria and kinesophobia (fear of exercise).
Now after a decade of so of biomedical research, and after research showing Graded Exercise therapy worked was discredited, we have a steady stream of studies showing different abnormalities and harmful reactions to exercise. Increased autoimmune activation post exercise, microclotting, mitochondial dysfunction, T-cell exhaustion. And most importantly with a dozen or so 2-day CPET studies, we have definitive proof that while healthy controls improve exertional capacity by exercising, these patients are the exact opposite, they worsen.
There’s even been a couple cases of young people 20-30 having a degenerative disease state that killed them.