Periapical periodontitis (also termed apical periodontitis, AP, or periradicular periodontitis) is an acute or chronic inflammatory lesion around the apex of a tooth root which is usually caused by bacterial invasion of the pulp of the tooth. The term is derived from peri- meaning "around", apical referring to the apex of the root (the tip of the root), and -itis meaning a disease characterized by inflammation. Periapical periodontitis can be considered a sequela in the natural history of dental caries (tooth decay), irreversible pulpitis and pulpal necrosis, since it is the likely outcome of untreated dental caries, although not always. In some cases, periapical periodontitis can occur due to occlusal high spots post-restoration, endodontic root filling material extrusion or bacterial invasion and infection from a gingival communication (rather than a pulpal source). Periapical periodontitis may develop into a periapical abscess, where a collection of pus forms at the end of the root, the consequence of spread of infection from the tooth pulp (odontogenic infection), or into a periapical cyst, where an epithelial lined, fluid filled structure forms.