Tooth decay is the destruction of tooth enamel and underlying tooth structure by acids — the disease dentists call dental caries. The acids come mainly from oral bacteria feeding on sugars and fermentable carbohydrates in the diet: the bacteria produce acid that demineralises enamel, and repeated attacks without remineralisation open cavities. It is one of the most common diseases in the world and almost entirely preventable. Fluoride — in toothpaste, water, or professional treatments — reharden enamel; limiting sugary foods and drinks, especially between meals, starves the bacteria; and regular brushing removes the plaque they live in. Once decay cavitates a tooth, only a dentist’s filling can repair it.