cavity or decay of a tooth, a localized disease that begins at the surface of the tooth and may progress through the dentine into the pulp cavity. It is believed that the action of microorganisms in the mouth on ingested sugars and carbohydrates produces acids that eat away the enamel. The protein structure of the dentine is then destroyed by enzymatic action and bacterial invasion. Diet, general health, structural defects of the teeth, and heredity affect one's chances of developing caries.

There are an estimated 600 species of bacteria that normally inhabit the human oral cavity. Thus, the bacterial composition of the oral cavity is suspected to play an important role in the development of caries and gum disease. In 2008 the discovery of a bacterial species named Prevotella histicola, which is present in both healthy and cancerous oral tissues and which generates acidic metabolites, such as acetic acid and lactic acid, that can damage tooth enamel, underlined the need to better understand oral microorganisms and their role in tooth decay.

Treatment of caries includes attention to diet, often entailing the avoidance of sweets, and care of the teeth by cleansing and by restoring teeth that have cavities. The addition of sodium fluoride to fluoride-deficient municipal water supplies has been observed to reduce the incidence of caries by as much as 65 percent. The sealing of the biting surfaces of teeth with adhesive plastics has also greatly reduced the incidence of caries. Today scientists are investigating ways to influence or alter the bacterial composition of the oral cavity for the prevention and treatment of tooth decay.