Why does human bodies decompose after death automatically and why not when human is alive?

Question

Why does human bodies decompose after death automatically and why not when human is alive?

Answer 1

When you are alive, your body temperature keeps the flesh alive. When you die, you are subject to room temperature where bacteria can grow and rot the flesh. Just like cooking meat keeps the meat good, or keeping it in the freezer keeps it good, but let it get in room temperature too long and it rots.

Answer 2

A living person has an elaborate immune system which protects them from invading pathogens and native opportunistic organisms.  Decomposition of flesh is essentially caused by microbes "eating" it and thus while we are alive and our cells are getting the proper life sustaining nutrients and our immune systems block microbes from eating us.