Biting your nails is definitely annoying, but it's also potentially horrible for people's health, says Richard Scher, an expert on nail disorders at Weill Cornell Medical College. Fingernails are basically cesspools for germs. For one, a family of bacteria called enterobacteriaceae—which includes salmonella and E. coli—tends to thrive in the crevice beneath the tips of your nails, Scher explains. When you bite your nails, those bacteria end up in your mouth and gut, where they can cause gastrointestinal infections that lead to diarrhea and abdominal pain. Amazingly, you can even get an STD from biting your nails, as finger chompers are also far more susceptible to HPV, TIME reports.