The part of the nervous system that governs flexion is more dominant. When we are fatigued and when we sleep, flexion tends to take over. As someone ages and loses higher brain function, their bodies start to assume a more flexed position. A posture of extension is created by higher brain (cortical) function.
Chronic inflammation can fatigue the body. Over-sympathetic nervous system can do the same. Illnesses and conditions in the body are usually associated with over-sympathetic dominance and inflammation.
A tendency for infections reflects both of the above, as well as lowered immune system function, intestinal flora imbalances, digestive disturbances, hormonal imbalances, blood sugar dysregulation, etc.
A traumatic experience can break compensations and shift the body towards a state of chronic imbalance. Rheumatoid conditions are associated with systemic fungal candida.