Colds and flu aren’t the only things you can catch from biting your nails. As Time The journal notes, bacteria known as enterobacteriaceae are commonly found on hands and fingers after contact with an infected surface. This bacteria, which includes salmonella and E. coli, thrives under the crevices between the nail tips. If you bite your nails, this bacteria is then transferred to your mouth and intestines leading to gastrointestinal infections, including diarrhea, abdominal pain, and vomiting.
There is another bacterial infection common to nail biters – paronychia. This infection usually occurs on the surrounding skin cuticle or up the sides of the nail, and there is often inflammation around the affected site. Based on DermNet NZa bacterial infection caused by cracks in the skin mixed with Staphylococcus aureus, Streptococcus pyogense, Pseudomonas, and other pathogenic bacteria such as herpes viruses and yeasts.
https://www.thelist.com/758194/what-really-happens-to-your-body-when-you-bite-your-nails/ What really happens to your body when you bite your nails