The outer layer of your skin is mostly made of dead cells. It's cold, dry, exposed to the elements, constantly discarded, and has the wrong pH for most biochemi

The outer layer of your skin is mostly made of dead cells. It's cold, dry, exposed to the elements, constantly discarded, and has the wrong pH for most biochemical processes that are optimized for the inside of your body. It takes a fairly invasive pathogen to break that barrier.