An abcessed tooth is a painful infection at the root of the tooth or between the gum and a tooth. It's most commonly caused by severe tooth decay. Other causes of tooth abcess are trauma to the tooth, such as when it broken or chipped, and gingivitis and gum disease.
These problems can cause openings in the tooth enamel, which allows bacteria to infect the center of the tooth( called the pulp). The infection may also spread from the root of the tooth to the bones supporting the tooth.
Dr. James L. Johnson