Patient-friendly definition from the DentistsDirectory.org dental glossary.
Dental caries is the medical term for tooth decay — an infectious, progressive bacterial disease that destroys hard tooth structure. Specific bacteria (primarily Streptococcus mutans) metabolise dietary carbohydrates and produce organic acids as a byproduct. These acids dissolve the mineral content of enamel and dentin in a process called demineralisation. If this process is not interrupted, a cavity forms: a physical hole in the tooth that requires restoration.
Decay does not develop overnight. Before a visible cavity forms, early lesions exist as white spot lesions — cloudy, dull areas in the enamel where demineralisation has begun but the surface is still intact. At this stage, the process can be fully reversed with fluoride, which remineralises the weakened enamel. Once the enamel surface breaks down, however, remineralisation cannot close the hole, and a filling is necessary. Untreated, decay progresses through enamel into the softer dentin, where it spreads much faster, and eventually into the pulp, where it causes infection that requires root canal treatment or extraction.
Caries prevention centres on four factors: removing plaque daily through brushing and flossing; fluoride exposure through toothpaste, fluoridated water, or professional applications; limiting the frequency of sugar and acid exposure; and regular professional cleanings to remove calculus and detect early lesions. Dental sealants on children's molars are highly effective at preventing decay in the deep grooves of back teeth where brushing cannot reach.