Teeth are made up of two layers, enamel on the outside and dentin on the inside. Enamel is the part that you can see. Enamel is extremely hard, the hardest substance in your body. Throughout the day a layer of bacteria called plaque coats the teeth. This layer of bacteria produces acids, which eventually dissolve holes in the enamel and form a cavity.
Once a cavity is formed the bacteria has then gone into the dentin. Because dentin is not very hard the acid from plaque dissolves it very quickly. If the bacteria dissolve through the dentin the decay can and will reach the nerve. If this occurs the bacterial infection is now within the nerve and a root canal is needed to treat the problem.
Cavities usually form in between teeth or on the top of them. Plaque can easily collect in between teeth even though you brush every day. The failure to floss on a daily basis is a cause of cavities in between the teeth. Most people also have very small grooves on the tops of their teeth, if this is the case the grooves may be too small for the bristles of their toothbrush to clean them effectively causing cavities. The failure to floss on a daily basis is another cause of cavities in between the teeth.
Sealants are a varnish that is used to seal grooves in the teeth and prevent cavities. Most people have sealants placed on the surface of the teeth during childhood but they do eventually wear off and have to be replaced in order to retain their effectiveness. Most cavities treated on adults within the practice are within a place where a childhood sealant ahs worn off.
Pain in the tooth does not determine if a cavity is present. Many times a cavity will not cause pain until it reaches the nerve, if it gets to this point the tooth will then need a root canal, build up, and a crown to fix it. If the issue fixed when the cavity was small, it would only have needed a filling.
Because of the size and location of some cavities radiograph x-rays are needed at least once a year to detect these problems. If you have had several in the past X-rays may need to be taken more often. The main thing to remember is that the earlier a cavity is detected the less expensive it will be to treat.