What causes a cavity?

Your mouth is a busy place. Bacteria - tiny colonies of living organisms are constantly on the move on your teeth, gums, lips and tongue. Having bacteria in your mouth is a normal thing. While some of the bacteria can be harmful, most are not and some are even helpful. Certain types of bacteria, however, can attach themselves to hard surfaces like the enamel that covers your teeth. If they're not removed, they multiply and grow in number until a colony forms. More bacteria of different types attach to the colony already growing on the tooth enamel. Proteins that are present in your saliva (spit) also mix in and the bacteria colony becomes a whitish film on the tooth. This film is called plaque, and it's what causes cavities.