Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Gum (Periodontal) Disease ?
    ### Response:
    When gingivitis is not treated, it can advance to periodontitis (which means "inflammation around the tooth.") In periodontitis, gums pull away from the teeth and form "pockets" that become infected. The body's immune system fights the bacteria as the plaque spreads and grows below the gum line.
    