Tooth decay is progressive destruction of a tooth from exposure to dental plaque, a sticky film composed of a combination of bacteria, acid, food particles and saliva. Dental plaque coats teeth after meals and gradually erodes the structure of the teeth, leaving holes known as cavities.

A tooth is made up of several layers. Enamel composes the hard outer layer. Dentin is the next, softer layer. Pulp is the inner layer that contains the nerves and blood vessels. This is the living part of the tooth.

Tooth decay – also known as dental caries – occurs when one or more of these layers begin to gradually dissolve as a result of acid created when bacteria, particularly Streptococcus mutans, feed on food particles, especially bits of sugary and starchy foods. As layer upon layer of plaque accumulates, the substance hardens into tartar. This buildup of plaque and tartar irritates the gums and causes gingivitis and periodontitis.

Tooth decay strikes people of all ages, but is particularly prevalent in children. It is the single most common condition in children ages 5 to 17 years and affects 59 percent of children in this age bracket, according to the U.S. Centers for Disease Control and Prevention (CDC).

Adults also experience tooth decay. About 27 percent of adults ages 35 to 44 and 30 percent of adults age 65 and older have untreated cavities. Much of this decay is found in the edges of fillings often placed during childhood that have weakened, fractured and leaked around the edges due to years of wear and tear. As bacteria seeps into these crevices, decay results.

In addition, recession of the gums often occurs during the aging process and leaves the roots of teeth vulnerable to decay. A soft tissue (cementum) covers tooth roots and is more susceptible to decay. The majority of people over the age of 50 have this form of tooth-root decay, according to the American Dental Association (ADA).

Once a patient experiences tooth decay, additional cavities are more likely to follow. Bacteria tend to adhere better to fillings and other restorations than they do to smooth, natural teeth, so these areas become magnets for the acid that causes cavities. Cracks or gaps in fillings also allow bacteria and food to enter the tooth.