Gum disease results from a build-up of plaque on the teeth and gums, or more specifically the bacteria in plaque. If left unchecked, the plaque bacteria will attack the gums and the bone underneath them, often resulting in bone loss that leads to tooth mobility and then tooth loss as time progresses.
Plaque build -up is also very detrimental to the health of your other major organs; heart, kidney, liver etc., so it is essential that proper gum and tooth care is practiced. Really, periodontal disease is bad news, and as you age it can cause serious problems for you, so pay attention to consistent aches and swelling in the gums.