Causes, incidence, and risk factors:

Gingivitis is a form of periodontal disease. Periodontal disease involves inflammation and/or infection that results in destruction of the tissues that support the teeth, including the gingiva (gums), the periodontal ligaments, and the tooth sockets (alveolar bone).
  
Gingivitis is caused by the long-term effects of plaque deposits. Plaque is the sticky material that develops on the exposed portions of the teeth, consisting of material such as bacteria, mucus, and food debris. It is a major cause of dental caries. Unremoved plaque mineralizes into a hard deposit called calculus (tartar) that becomes trapped at the base of the tooth. Plaque and calculus cause mechanical irritation and inflammation of the gingiva. Bacteria, and the toxins produced by the bacteria, cause the gums to become infected, swollen, and tender.

Injury to the gums from any cause, including overly vigorous brushing or flossing of the teeth, can also cause gingivitis. The risks for development of gingivitis include uncontrolled diabetes, pregnancy (because of hormonal changes that increase the sensitivity of the gingiva), general (systemic) illness, and poor dental hygiene. Misaligned teeth, rough edges of fillings, and mouth appliances (such as orthodontic appliances, dentures, bridges, and crowns,) can irritate the gums and increase the risk of gingivitis. Medications such as phenytoin and birth control pills, and ingestion of heavy metals such as lead and bismuth are also associated with gingivitis.
  
Many people experience gingivitis to a varying degree. It usually develops during puberty or early adulthood and may persist or recur frequently.