An abscessed tooth is an infection caused by tooth decay, periodontal disease or a cracked tooth. These problems can let bacteria enter the pulp (the soft tissue of a tooth that contains nerves, blood vessels and connective tissue) and can lead to pulp death. When pus builds up at the root tip in the jaw bone, it forms a pus-pocket called an abscess. If the abscess is not treated, it can lead to a serious infection in the jaw bone, teeth and surrounding tissues.