Hop into a lab and pop the scrapings from under your nails into a petri dish. Grow a culture. Then come back and ask me again.

You should be washing your hands before you eat, but given that 70% of the population doesn't do that, and that only 13% of the remaining population use soap, it's unlikely that's happening.

>As long as you're not using nail-biting as a nervous habit

... for what other reason does one bite their nails regularly?

>and biting them past the tip of the finger, it shouldn't hurt the nail.

My comment was about the ingestion of fecal and other bacteria from unknown sources as it gets stuck under our nails in our daily lives. Door handles, light switches, debit machines, etc... When I work on people's vehicles, I wear gloves when handling their keys and anything inside of the vehicle because people are gross.

Also, cutting or biting a nail too short (which the definition of varies from person to person, and it isn't always to or past the edge of the bed) can cause growth issues depending on the frequency and mechanism of damage.

>And putting your finger in your mouth is extremely common.

It's a habit which ought to have been broken by the time one has hit adolescence. It shouldn't be extremely common, especially given the rapid mutation of communicable diseases right now.