These are factors either local or general.
Local factors
Most of the time gingivitis due to a local cause: poor oral hygiene, the presence of tartar, teeth apposition, fillings (amalgam and / or composite) old dental crowns poorly adapted and / or presence of fixed braces, which retain more easily the plate, make brushing less effective and thus the accumulation of bacterial plaque. With the plaque, there is a gradual increase in bacteria content increasingly destructive, which together produce toxins. These toxins accumulate and eventually cause the formation of pus bags, the "pockets", then there is attack oral tissues and trigger the body's inflammatory response.
The general factors 
It all factors that will affect the defense response of the body (inflammatory / immune). They make the gums more susceptible to attacks of tartar or plaque bacteria. At the slightest irritation, the gum into flames. These include: hormonal factors (during puberty, rules, pregnancy, menopause…), smoking, stress and certain diseases of the immune system (the AIDS virus, hepatitis…). In addition, in metabolic diseases like diabetes, there is an imbalance of flora mouth, components of saliva, leading to the development of gingivitis. Finally, they may also be due to a deficiency of vitamins. It is therefore necessary, in the presence of these factors, not let the elements settle irritants, i.e. to eliminate bacterial plaque even more drastic and more frequent cleaning. Periodontitis : When gingivitis is not treated, in the longer term, it will evolve into periodontal disease: periodontitis. The bacterial plaque that has accumulated will migrate through the gingivodentaire path to the periodontal ligament and alveolar bone. Microbes that bacterial plaque, known as gum, are, first, destroying the tissue supporting the teeth (ligament and bone) under the action of toxins and enzymes they produce, and secondly, turn into tartar in gingival drive that increase in the phenomenon of mechanical destruction. It should be noted that smoking is considered a major risk factor for the development of periodontitis.