For the healthy gu,. the periodontal ligament attaches the rooths of the teeth to the alveolar bone. 

By-products of toxin, mildew and other dental plaque stimulate gums, which will cause swollen and bleeding gum.Then the periodontal diseases develops further. 

When dental plaques destroy periodontal tissues, the gingivitis comes into being.Then it develops into periodontal pocket. The formation of periodontal pocket is more favorable to dentral plaque buildup and retention, which will cause more severe inflammation and consequent exposed tooth rooth and sensitive tooth. 

In the periodontitis progressive stage, the teeth lose firm support due to the dental ligament and alveolar bone are being destroyed continuously. If not treated timely, the affected teeth by the inflammation will become lossen and even fall off.