Tooth decay and implants, dental model. This model shows the teeth of the left lower jaw of a human. This model has transparent gums, showing the roots of the teeth. From far right, the teeth and their conditions are: wisdom tooth (tooth decay, black), second molar (healthy), first molar (cracked; tooth decay; infected root and inflamed nerve pulp, red), second pre-molar (filled with a dental metallic-looking filling), first pre-molar (has been replaced with a screw-based implant), canine (healthy), lateral incisor (broken), first incisor (healthy). 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.