Tooth decay is not a new problem. As mentioned in a previous article, the ancients believed that a worm caused the pain in your tooth. Of course, this is not the case. Tooth decay is defined as a thinning out and eventual collapse of the tooth enamel. The reason this happens is because the chemicals that break down the minerals in your enamel become outweighed by the bacteria that cause tooth decay. You can see that this is a complex medical condition, and there are many factors at play.