Causes

Cavities are caused by tooth decay — a process that occurs over time. Here's how tooth decay develops:

• Plaque forms. Dental plaque is a clear sticky film that coats your teeth. It's due to eating a lot of sugars and starches and not cleaning your teeth well. When sugars and starches aren't cleaned off your teeth, bacteria quickly begin feeding on them and form plaque. Plaque that stays on your teeth can harden under or above your gum line into tartar (calculus). Tartar makes plaque more difficult to remove and creates a shield for bacteria.
• Plaque attacks. The acids in plaque remove minerals in your tooth's hard, outer enamel. This erosion causes tiny openings or holes in the enamel — the first stage of cavities. Once areas of enamel are worn away, the bacteria and acid can reach the next layer of your teeth, called dentin. This layer is softer than enamel and less resistant to acid. Dentin has tiny tubes that directly communicate with the nerve of the tooth causing sensitivity.
• Destruction continues. As tooth decay develops, the bacteria and acid continue their march through your teeth, moving next to the inner tooth material (pulp) that contains nerves and blood vessels. The pulp becomes swollen and irritated from the bacteria. Because there is no place for the swelling to expand inside of a tooth, the nerve becomes pressed, causing pain. Discomfort can even extend outside of the tooth root to the bone.