Shock

Shock is a byproduct of lack of blood circulating through the body.  Many different problems can cause shock but the most well known is the fear response.  As I have said before one of the first reactions to the body moving into the 'fear state' is increased blood flow and moving blood away from non-essential areas.  Areas like extremities (hands and feet).  This is formally known as vasoconstriction with a narrowing of the blood vessels resulting in a lack of blood circulating.  What the body is trying to do is force the creation of adrenaline hormones, 'shocking' the body into action.

So all in all shock like fear is ultimately a defense mechanism, but this form comes with some nasty side effects.  It is actually a life-threatening emergency sometimes resulting in uncontrolled bleeding, severe burns and spinal injury.  Every person reacts differently to rapid influx of hormones into their system.  Some take it extremely poorly and the body can't handle the sudden change of internal environment.  This along with the 'fear state' already in effect leaving their immune system compromised results in a backlash of fatal proportions.