It is called Gum disease. If plaque is not carefully brushed away, it hardens to form a calcified deposit called calculus, or tartar, which may inflame the gums and make them pull away from the tooth. This results in a gap between tooth and gum where trapped food provides a feast for bacteria that may infect your gums. The dentist can help control this condition, but if this is not cared for, the tissue surrounding the teeth may become so damaged that the teeth actually fall out. More teeth are lost this way than through tooth decay.
