A cavity usually starts long before it's identified in the dentist's office. In fact, you can probably trace its origins back to the moment you started neglecting your teeth. When you don't take care of your teeth with regular brushing and flossing, the sugars and food particles that are left in your mouth break down and provide nourishment for the bacteria that are always present there. Those bacteria subsequently experience a population boom, and that's where the road to a cavity begins.
There are about 100 types of bacteria in your mouth, but the problematic ones are Streptococcus mutans and Lactobacilli [source: Colgate].While other oral bacteria can be useful, Streptococcus mutans and Lactobacilli convert refined sugar and carbohydrates into acids that eat away at tooth enamel.
As bacteria multiply, they interact with food particles or sugars that are left on a tooth, causing the latter to become acidic. This contributes to the breakdown of that tooth's enamel.
As the enamel wears away (a process that can take anywhere from months to years), a hole develops, and this is the start of your cavity [source: Korwin]. At that point, you'll probably start paying a little more attention to your mouth, but for all the wrong reasons. The effected tooth will become sensitive to touch, as well as to heat and cold. It may hurt when you bite down. Pus may even begin to form around it.
Left unchecked, a cavity will get worse and eventually destroy the entire tooth. Once decay penetrates the enamel, it attacks the dentin, the second layer of the tooth that surrounds the pulp and has nerve networks woven throughout it. Then it exposes the pulp, which is connective tissue riddled with blood vessels at the core of the tooth. Once the pulp decays, you're on the road to tooth loss. Everyone around you may also find that your breath is pretty fierce.
Ideally, though, you wouldn't let a cavity get to this point -- you'd see a dentist and take measures to prevent tooth loss. In that sense, you can stop a cavity from getting worse. However, some sources may suggest that you can treat a cavity through re-mineralization without a dentist's intervention. Find out if there's any truth to this on the next page.