Periodontal disease is marked by bacterial overgrowth. However, a persistent immune response to chronic infections in the mouth is believed to play a major role in gum destruction.

Bleeding of the gums, even during brushing, is a sign of inflammation and the major marker of periodontal disease. One exception is juvenile periodontitis, in which symptoms are mild or even absent. It should be noted that the gums of smokers with periodontal disease tend to bleed less than nonsmokers.

Smoking is the single major preventable risk factor for periodontal disease. The habit can cause bone loss and gum recession even in the absence of periodontal disease. A number of studies indicate that smoking and nicotine increase inflammation by reducing oxygen in gum tissue and triggering an over-production of immune factors called cytokines (specifically ones called interleukins), which in excess are harmful to cells and tissue.

Furthermore, when nicotine combines with oral bacteria, the effect produces even greater levels of cytokines and eventually leads to periodontal connective tissue breakdown. Studies suggest that smokers are 11 times more likely than nonsmokers to harbor the bacteria that cause periodontal disease and four times more likely to have advanced periodontal disease.

As the disease advances the gums recede, and supporting structure of bone is lost. Teeth loosen, sometimes causing a change in the way the upper and lower teeth fit together when biting down or a change in the fit of partial dentures.

Gingivitis, in varying degrees, is nearly a universal finding in children and adolescents. In rare genetic cases, children and adolescents are subject to destructive forms of the disease. Researchers have also observed some of the organisms seen in periodontal disease in young children without signs of gum problems.