Cavities are caused by acid erosion of tooth enamel. Many different types of bacteria normally live in the human mouth. They accumulate (along with saliva, food particles and other natural substances) on the surface of the teeth in a sticky film called plaque.

Plaque forms especially easily in cracks, pits or fissures in the back teeth; between teeth; around dental fillings or bridgework; and near the gum line. Some of the plaque bacteria convert sugar and carbohydrates (starches) in the foods we eat into acids. These acids dissolve minerals in the surface of the tooth, forming microscopic pits or erosions that get larger over time.
The damage can occur anywhere the tooth is exposed to plaque and acid, including the hard outer enamel on the tooth crown or the unprotected root of the tooth that has been exposed by gum recession. Caries can penetrate the protective enamel down to the softer, vulnerable dentin (main body of the tooth) and continue through to the soft tooth pulp and the sensitive nerve fibers within it. No one really knows what causes the pain associated with cavities, but theories include inflammation caused by bacteria, exposure of the root surface and an imbalance of fluid levels in the tubules inside the dentin.