Dental disease is caused by the combination of food residue and the bacteria living in the mouth, though it can be complicated by things such as fractured teeth.
Grade I: Plaque has formed on the teeth, and the gum tissue (gingiva) at the edge of the tooth is inflamed.
Grade II: All of the gingiva is swollen and inflamed. Tartar has continued to form.
Grade III: The gingiva is fragile, and may bleed. It is beginning to recede from the edge of the tooth.
Grade IV: Chronic infection is destroying the gingival tissue, the root of the tooth, and the bone the teeth are attached to.