Technically known as Dental Caries, tooth decay is second only to Periodontal disease as the most common of all diseases in humans. There are two stages to tooth decay. The actions of bacteria cause both.
The outer layer of the tooth is composed of enamel, the hardest substance in the human body. The enamel layer is composed of long rods of Calcium Hydroxyapatite crystals bonded together by a partially mineralized organic matrix. The structure is much like a bundle of pencils glued together side by side, with one end of the bundle at the surface and the other at the deeper dentin layer.
Enamel decay is mostly a process of dissolving the enamel rods with acid. The acid is a waste product of the metabolism of sugars by certain anaerobic bacteria. Anaerobic bacteria do not use oxygen in their metabolism, so they are unable to fully metabolize sugars (sucrose or raw sugar, galactose, fructose, and other complex sugars which appear in everything from candy and fruit to milk and semenal fluid.) to carbon dioxide and water as higher life forms do. Instead they break them down to lactic and pyruvic acids and release them. (This is the same process that occurs in your own muscles when you exert yourself. We use anaerobic metabolism as the first stage of metabolizing sugars. When this stage of metabolism gets ahead of the second, slower aerobic stage, the same acids accumulate in our muscles and cause the 'burn' of over exertion.) Because the enamel rods are aligned from the surface toward the center of the tooth, enamel decay progresses narrowly but directly toward the deeper layers of the tooth. The initial point of penetration may be no wider than a pinhole.
Once the initial penetration to the inner dentin layer has been made, the nature and direction of the decay changes. The dentin is also largely composed of Calcium Hydroxyapatite, but the structure is much more diffuse and the organic portion is greater. Acid is still the primary culprit, but other enzymatic processes are involved in destroying the organic component of the dentin. From the initial point of penetration, decay spreads rapidly and uniformly in all directions, undermining the hard, but brittle, enamel. The cavity first becomes visible to the patient when the undermined enamel collapses into the void left by the destroyed dentin. It appears to have occurred overnight, but the enamel decay probably took over a year and the dentin process almost as long.
By the time a cavity becomes visible to the patient, it may already be too late to repair the damage with a simple filling and a crown and possibly a root canal filling may be necessary. This is the reason dentists recommend clinical examinations every six months and cavity detecting x-rays annually for most patients.