A tooth abscess is a pocket of pus that’s caused by a bacterial infection. The abscess can occur at different regions of the tooth for different reasons. A periapical abscess occurs at the tip of the root, whereas a periodontal abscess occurs in the gums next to a tooth root. .A periapical tooth abscess usually occurs as a result of an untreated dental cavity, an injury or prior dental work. Leaving a tooth abscess untreated can lead to serious, even life-threatening, complications.