An accumulation of pus that is enclosed in the bone tissue at the tip of the root in a tooth. The cause of a tooth abscess is usually a bacterial infection in the pulp. The infection may originate in untreated tooth decay or in dead pulp tissue inside the tooth. If there is bacteria in gum tissue that has severely receded or if bacteria remain in the gum surrounding the root of a tooth following a root canal treatment, the bacteria may produce an abscess.