Tooth decay usually happens slowly over a period of months or years.

Decay begins when bacteria in your mouth increase during the first 20 to 30 minutes after you eat. The bacteria make acids, which eat away at the hard mineral layers of the tooth. A hole (cavity) forms when the acids cause more damage than the tooth can repair.

See pictures of a tooth and tooth decay.

A tooth has an outer layer (enamel), a middle layer (dentin), and a center (pulp). The more layers that are affected by decay, the worse the damage.

• When tooth decay is mild, the area of decay is small and has not pierced the tooth surface. You can sometimes stop the decay with improved care, such as having your dentist apply fluoride to your teeth.
• When tooth decay gets worse, a cavity forms. You will need a filling to stop the decay and prevent more damage.
• If the pulp begins to decay, the tooth will likely die, because the pulp contains nerves and blood vessels that supply the tooth. After a decayed tooth dies, an abscess may form in the bone at the end of the root. For more information, see the topic Abscessed Tooth.

Types of cavities (dental caries) are:

• Pit and fissure cavities, which form in the deep pits and grooves on the chewing and biting surfaces of the back teeth.
• Smooth-surface cavities, which form on the sides of teeth, including between the teeth.
• Root cavities, which form on the root and can extend below the gum line. Root decay is less common than decay in other parts of the tooth. But root decay is more likely to damage the tooth pulp.
• Recurrent or secondary cavities, which form where you already had a cavity.

Untreated tooth decay causes more severe problems and can lead to gum disease. For more information, see the topic Gum Disease.

Your saliva helps prevent tooth decay. It reduces acid damage to a tooth by washing away sticky, sugary foods that feed bacteria. The minerals in saliva also can help repair the tooth.