Tooth decay is a common tooth ailment associated with people who do not observe good dental hygiene. What Happens? Bacteria multiply in your mouth and start eating away at your teeth. A tooth has three distinct layers: the outermost, enamel; the middle layer, dentine; and the innermost, pulp cavity. Bacteria start eating away at your enamel. At this time, you may not feel anything, but you might notice food sticking on the top surface of your tooth, where a hole may have started forming.