Tooth decay is one of the most common infectious diseases known to man. (Only the common cold is more prevalent.) Decay or caries occurs when the teeth are attacked by acid, which is introduced to the mouth in one of two ways: 1) Through the consumption of foods or acidic drinks like soda pop, fruit juice, etc. or 2) Through the breakdown of the sugars and starches in our diet by the naturally occurring bacteria in our mouths.
For most people, cavities develop due to excessive ingestion of sugary foods and candy, as well as soda pop. This causes the localized acid load in the mouth too increase to such a high level (pH < 5.5) that the acid literally rots a hole in the tooth. Excessive consumption of carbonated beverages like colas can be especially bad for the teeth, because they contain both acid and sugar.
To avoid cavities, and still enjoy a sweet or carbonated beverage, simply brush and floss immediately after eating or drinking these. This removes the sugars and acids and stops the decay process from progressing.
However, for some of us, the decay process has already occurred, necessitating a filling or perhaps a crown.