Also known as tooth decay, a cavity is the deterioration of the tooth structure caused by acids in bacterial plaque. The decay can damage both the dentin (the inner part of a tooth) and the enamel (the outer portion of a tooth). The process of decay starts when foods containing carbohydrates (starches and sugars) are left on the teeth. The bacteria in plaque consume the food and produce acid as a by-product. The acid erodes the enamel, allowing the bacteria to attack the dentin inside the tooth.
When bacteria penetrate sufficiently deeply inside a tooth, sensitivity and pain can result. If detected and treated early, decay will not destroy a large amount of tooth structure and usually is not painful. If not detected early, and allowed to progress, the bacterial invasion can reach the dental pulp, resulting in a painful abscess.