I used to bite my nails too, until someone suggested I look at what fingernail scrapings (that would be that stuff under your nails) look like under a microscope and that killed the desire. Trust me, after you see that, you never want to chew your nails again.
I can only imagine. People rarely wash their hands properly, so I can speculate that there is at least fecal matter and dead skin cells among whatever other goodies are festering in there.
I watched a video in my biology class last term about the microbes that basically feast on your discarded nail clippings.
