Tooth decay refers to loss of minerals from tooth structure caused by bacteria.
Animation of the longitudinal section of a tooth crown with decay started in enamel and then all the way to dentine leading to apparent cavity. Photograph of the lower right deciduous teeth, one of which has developed an apparent cavity.

Why does a tooth become decayed?
Animation of the longitudinal section of a tooth crown with the acids produced by bacteria, resulting in enamel discolouration indicating mineral loss from the enamel. The tooth surfaces are frequently covered with dental plaque. Every time food or drinks are consumed, the bacteria in the dental plaque will metabolise the sugars in the food to produce acids, which will demineralize the tooth surfaces.
Animation of the longitudinal section of a tooth crown. It is protected by saliva, resulting in the colour of enamel turning white indicating the halting of mineral loss and the replenishment of minerals. Although saliva can neutralize the acids and slow down the demineralization process, it takes a long period of time to do so.
Animation of the longitudinal section of a tooth crown with the acids continuously produced by bacteria in a short period of time resulting in the continuous discolouration of enamel indicating that the mineral loss is continuous and the tooth decay spreads to dentine. If we eat and drink frequently, the acid cannot be adequately neutralized by the saliva, and the continuous demineralization will eventually result in the formation of tooth decay.
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