The process by which teeth are broken down by the action of bacteria in the mouth. Three factors must coincide if dental decay is to occur: the presence of bacteria, the presence of food (sugar) for the bacteria, and a set of teeth which is susceptible to the effects of bacterial products. Bacteria which are always in the mouth, such as certain streptococci, convert sugars into acids which attack tooth enamel. Dental decay or caries is characterized by sudden discoloration, softness, and, subsequently, loss of tooth tissue, with the appearance of the characteristic small cavities. If not treated, these cavities progressively increase in size, and intense pain and complications, such as inflammation at the tip of the root, can arise. Caries is treated by the usual dental procedures, with each site where decay is in progress being treated directly. But the emphasis must also be on prevention, I.e. minimizing the three factors mentioned above. Thus, a restriction in the intake of sugars on which the bacteria feed has a very positive effect. Chocolate, jam, treacle, etc. are particularly injurious. Good oral hygiene and proper brushing of the teeth limit both the amount of bacteria and the quantity of food which the bacteria have at their disposal. In addition, the sensitivity of the teeth can be reduced by the use of fluoride. If fluoride is incorporated in the enamel it increases resistance to the acids produced by the bacteria. This has been shown by research in areas where fluoride occurs naturally in drinking water, and in areas where the drinking water has been fluoridated. A considerable reduction in caries is the result in both cases. Fluoride is best administered in the form of tablets from birth onwards, and by application of fluoride by the dentist or dental hygien-ist. Toothpaste containing fluoride should also be used. Some people advise against giving tablets and toothpaste to young children every day, because stains can then later arise on the teeth (mottled enamel).