A tooth abscess sounds like a complex technical term, but it basically means the abscess underneath the affected root and the damaged nerve is full of pus. Caused by a bacterial infection, an abscess can form in different parts of a tooth. One cause of a painful toothache is an abscessed tooth. An abscess is an infection in or around the root of the tooth, which may or may not be painful. It occurs when the pulp, the soft tissue inside the root canal, dies and becomes inflamed. Learn more about what causes a tooth abscess and why it is important to get it treated.