According to the vagus nerve infection hypothesis (VNIH), there is a physiological reason behind post-exertional malaise: Exercise provokes muscle tissue to produce the proinflammatory cytokine IL-6, which would then exacerbate the ongoing local cytokine response within vagus nerve ganglia or paraganglia. That's the hypothesized mechanism behind post-exertional malaise.
The CBT practitioners in the infamous PACE study were focused on avoidance/fear of activity because they began with the assumption that CFS is psychological. They think the fear of activity itself is the cause of CFS; I'd say that fear of activity is justified
- See more at: http://simmaronresearch.com/2014/02/michael-vanelzakker-ph-d-talks-vagus-nerve-infection-hypothesis-chronic-fatigue-syndrome-mecfs/#sthash.tyuWTJH1.dpuf