The infection damages the tissue that connects the gum to the roots of the tooth. Once the tissue has been destroyed the gum pulls away from the tooth, this forms a pocket which bacteria thrive in. If not treated, in time the bone anchoring the teeth in the jaw will dissolve, making the teeth loose and eventually the teeth will fall out.
Periodontal disease is irreversible, but you can slow down the progression by regular visits to the Dentist and Hygienist also improving your oral hygiene at home.