Tooth decay. Close-up of the rotten upper molar teeth of a patient suffering from dental caries. Caries is the decay and crumbling of the teeth. It is caused by the 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. High levels of sugar in the diet and poor dental hygiene are the main causes of caries. Once the decay has spread to the dentine (the hard tissue beneath the tooth enamel), as seen here, caries is treated by removing the damaged parts of the teeth and replacing them with fillings.