An abscessed tooth develops as a result of tooth decay and/or gum disease.  These two problems then lead to an infection which then in turn leads to an abscessed tooth. 

There are several problems that can initially lead to an abscessed tooth including: experiencing trauma to a tooth, whether it is broken and/or chipped, a cavity that has been left untreated, and/or gum disease.

If a tooth is damaged and left untreated, the inside of the tooth which is known as the pulp may become infected. This infection then allows for bacteria to spread from the tooth to the tissue and the surrounding area of the tooth. This is how the abscess is formed. If an abscessed tooth develops as a result of gum disease, the gum disease itself causes the gums to pull away from the teeth forming gaps between the teeth and gums. These gaps allow for food particles to build up enabling bacteria to grow and creating the abscess.

If an abscess is left untreated the infection can cause the bone around the tooth to dissolve. The end result is that the tooth becomes loose which may result in the tooth having to be pulled.