A cavity is a small hole that forms inside the tooth because of tooth decay. Cavities are formed when plaque build-up on the outside of the tooth combines with sugars and starches in the food you eat. This produces an acid that can eat away the enamel on your tooth. If a cavity is left untreated, it can lead to more serious oral health problems. Cavities can be prevented by remembering to brush your teeth at least three times a day and floss between teeth at least once.