Tooth decay is a kind of chronic infection of the hard tissue of teeth, which is mainly caused by poor oral hygiene, because of not brushing and rinsing carefully, so that the food residues are stored on the tooth surface, the bacteria in the mouth will decompose the food residues and produce acidic substances, the acidic substances stay on the tooth surface for a long time, it will make the hard tissue of teeth demineralize and become soft, and finally form cavities. Some people have bad eating habits, like to drink drinks or cakes, cookies, candies and other sweets, which are more likely to produce more acidic substances and lead to rapid development of tooth decay. Some people have poor salivary secretion function in the mouth, so the saliva volume decreases and the saliva viscosity increases, because the saliva inside the mouth loses the flushing function, it also makes the caries progress more drastically than ordinary people. If the enamel development is poor, the hardness and thickness of the enamel decreases, which also makes it easy to suffer from caries.