Etiology The currently accepted doctrine of caries etiology is the quadruple factor doctrine, which mainly includes bacteria, oral environment, host (i.e. refers to parasitic organisms including parasites, viruses and other parasitic organisms on it) and time. The basic point is that the cariogenic food sugar (especially sucrose and refined carbohydrates) adheres firmly to the tooth surface and an acquired film formed by salivary proteins. This acquired film not only can firmly adhere to the tooth surface, but also can have enough time to produce acid in the deep layer of plaque under the suitable temperature, invade the tooth, demineralize it, and then destroy the organic matter to produce cavities. 1.Bacteria is the necessary condition for the occurrence of caries, generally speaking, there are two types of caries-causing bacteria, one is acid-producing bacteria, mainly Streptococcus pyogenes, Actinomyces spp. and Lactobacillus, which can make carbohydrates decompose and produce acid, leading to the demineralization of inorganic material of teeth; the other is Gram-positive cocci, which can destroy organic matter, and can make teeth form cavities after long-term action. 2.Oral environment Oral cavity is the external environment of teeth, which is closely related to the occurrence of caries, among which the leading role is mainly played by food and saliva. (1) Food is mainly carbohydrate, which is related to the formation of plaque matrix as well as the main energy source of bacteria in plaque. Bacteria can use carbohydrate (especially sucrose) to metabolize and produce acid, and synthesize extracellular polysaccharide and intracellular polysaccharide, the organic acid produced is beneficial to the growth of acid-producing and acid-resistant bacteria, and also beneficial to the demineralization of dental hard tissue, polysaccharide can promote the adhesion and accumulation of bacteria on the tooth surface, and in provide a source of energy when exogenous sugar is lacking. (2) Saliva has the following roles under normal conditions: mechanical cleaning; antibacterial; antacid; and antisoluble. When the quantity and quality of saliva change, it can affect the caries rate, and the caries rate of patients with dry mouth or salivary secretion is obviously increased clinically. Patients with maxillofacial radiation therapy may have multiple carious teeth because salivary glands are destroyed; on the other hand, when the amount of lactic acid in saliva increases, it also favors the occurrence of caries. 3.Host The tooth is the target organ in the process of caries, and the morphology, mineralization degree and tissue structure of the tooth have direct relationship with the occurrence of caries. 4.Time The occurrence of caries has a long process, from the initial caries to the clinical formation of cavity usually takes 1.5~2 years, so even if the cariogenic bacteria, suitable environment and susceptible host exist at the same time, caries will not occur immediately, only when the above three factors exist at the same time for quite a long time, then caries may be produced.