Here's some interesting information on the Purple Heart and it's standards.
http://www.americal.org/awards/ph.htm
(5) Examples of injuries or wounds which clearly do not qualify for award of the Purple Heart are as follows:
(a) Frostbite or trench foot injuries.
(b) Heat stroke.
(c) Food poisoning not caused by enemy agents.
(d) Chemical, biological, or nuclear agents not released by the enemy.
(e) Battle fatigue.
(f) Disease not directly caused by enemy agents.
(g) Accidents, to include explosive, aircraft, vehicular, and other accidental wounding not related to or caused by enemy action.
(h) Self-inflicted wounds, except when in the heat of battle, and not involving gross negligence.
(i) Post traumatic stressdisorders.
(j) Jump injuries not caused by enemy action.
But as you can see there are "exceptions".
http://www.3rdmarines.net/Vietnam_Se...rple_Heart.htm