To understand periodontal disease, I like to use the analogy of a splinter. If you had a piece of wood stuck in your hand and left it there, it would be a matter of time when the area would get infected. That is your immune system working trying to get rid of the splinter. What is happening is your immune system makes the infection that surrounds the splinter and destroys the tissue adjacent to the splinter. After enough destruction occurs, the splinter falls out and the area heals.
In our mouth, plaque coats our teeth and the bacteria in the plaque work their way under the gum making our teeth act like splinters. If left undisturbed, your immune system makes infection that slowly destroys the tissue holding your tooth, including the bone, around the teeth. Eventually, the tooth gets loose and falls out.
Prevention of periodontal disease involves cleaning our teeth thoroughly every day. This involves cleaning between the teeth and slightly under the gums. Professional cleaning every three months is recommended for people with periodontal disease.