Gangrein can not be passed because this is not a virus or bacteria per se, it is a bodies natural progression. When a part of a persons anatomy looses blood circulation and therefore proper oxygen and removal of toxins naturally generated by the cells that compose the body, the cells start to die and atrophe. this is a laymans explanation (and may not be entirely accurate, but pretty close i am sure.) of it. Gross to be sure, but no worse than any other part of this funky experiance we call life :P