Tooth decay, dental model. This model show the teeth of the upper and lower left jaws of a human. This model has transparent gums, showing the roots of the teeth. Tooth decay (black) is seen in one of the pre-molars (centre, upper jaw), in a wisdom tooth (upper right) and in a molar (lower right). This molar has cracked, and the decay has inflamed the tooth's nerve pulp (red). An impacted wisdom tooth (partially seen, lower right) has caused another area of inflammation. Tooth decay is caused by acid-secreting bacteria feeding on food debris in the mouth. The infected area can be drilled out and filled, or the tooth can be replaced completely.