The mouth, or oral cavity, which comprises the lips, tongue, teeth, gums, and related structures, is subject to a large number of disease processes. Periodontitis, an inflammatory disease of the tissues supporting and surrounding the teeth, and dental decay are the most common diseases; together they account for almost all tooth loss. Other diseases of the mouth can be classified as cysts; diseases of the salivary glands; keratotic, inflammatory, ulcerative, and proliferative lesions; oral infections; and an unusual form of rapidly destructive periodontitis observed in some patients with acquired immune deficiency syndrome (AIDS).