What Causes It?

Gingivitis develops as plaque and calculus irritate the gums, making them tender, swollen and likely to bleed easily.

Periodontal Disease occurs when the irritation continues, affecting tooth anchoring tissues.  Pockets form that collect plaque.  Roots are exposed and teeth can become sensitive to cold and touch.

In advanced Periodontal Disease, teeth lose more support as the disease continues to destroy the periodontal ligaments that anchor the tooth to bone.  Left untreated, affected teeth become loose and fall out.

More Info:  http://www.mouthhealthy.org/en/az-topics/p/Periodontics.aspx