In order to understand gum disease, we need to first understand the structures surrounding the tooth. The tooth is divided into the crown and the root.
The root is surrounded by bone, which in turn is covered by gums. There is a space between the tooth and gums, called a pocket, which is present both in healthy and diseased states.
Gum disease originates from bacteria that gain entry into the pocket. When bacteria enter, they attack the gums. This causes injury to the gums, resulting in spontaneous bleeding upon touch, pain, redness, and sometimes swelling.