A periapical abscess is a pocket of infection (pus) around your tooth root. This type of abscess forms when harmful bacteria from your mouth invade your tooth pulp. (This can happen if you have a cavity or a crack in your tooth.) The infection can spread all the way to the tip of the root and into the surrounding tissues. Left untreated, a periapical abscess can spread to other parts of your body and cause serious, life-threatening complications. If you think you have an abscess, you should seek care immediately.