Periodontal pathology is a science or a study of periodontal diseases. Periodontal diseases can affect one or more of the periodontal tissues/structures (e.g. alveolar bone, periodontal ligament, cementum and gingiva). While there are many different periodontal diseases that can affect these tooth-supporting tissues/structures, by far the most common ones are plaque-induced inflammatory conditions, such as gingivitis and periodontitis. Often the term periodontal disease or gum disease is used as a synonym for periodontitis, specifically chronic periodontitis. While in some sites or individuals, gingivitis never progresses to periodontitis, data indicate that periodontitis is always preceded by gingivitis.