What is an abscessed tooth? An abscess is a pus-filled sac caused by an infection when the decay in a tooth has done substantial damage and the cavity has gone very deep into the tooth. A fractured tooth or root can also let bacteria in and cause infection that leads to an abscess. An abscess can be extremely painful, and even life-threatening. Many times, the best treatment for an abscess is the extraction of the infected tooth. After the extraction and ample healing time, the missing tooth can be replaced with a dental implant or a bridge.