Tooth decay. Rotten teeth and receding gums of a patient suffering from dental caries and periodontal disease. Caries is the decay and crumbling of the teeth. It is caused by bacteria in plaque, a substance which covers the surface of unclean teeth. The bacteria break down sugar in the mouth, producing an acid which progressively destroys the teeth, causing pain, discolouration and, eventually, tooth loss. These bacteria also cause periodontal disease, the erosion of the tissues that attach the teeth. In its advanced form spaces develop between the gums and the teeth (for example at far left). Poor dental hygiene is the main cause of both of these conditions.