Dr. Lee to Nina: "Gingivitis is caused by buildup of bacterial colonies in your mouth, as illustrated here, which causes mild infection. Part of the body’s response to infection is pain and swelling. Recent changes in your hormones from the oral contraceptive may also affect the sensitivity of your gums."
Studies both in vitro and in vivo have shown that gingivitis is an inflammatory process caused by poor oral hygiene; gingival abrasion; stress; general illness; uncontrolled diabetes; smoking; or hormonal changes, such as during pregnancy or puberty.
Plaque biofilm formation is the first phase of the process. The microorganisms that form biofilm are almost entirely gram-negative bacteria. Inadequate interdental plaque removal allows a thick layer of bacteria to accumulate.