It's thought that periodontitis begins with plaque — a sticky film composed mainly of bacteria. Plaque forms on your teeth when starches and sugars in food interact with bacteria normally found in your mouth. Brushing and flossing your teeth removes plaque. But plaque re-forms quickly, usually within 24 hours.
Plaque that stays on your teeth longer than two or three days can harden under your gumline into tartar (calculus). Tartar also may form as a result of the mineral content of your saliva. Tartar makes plaque more difficult to remove and acts as a reservoir for bacteria. You can't get rid of tartar by brushing and flossing — you need a professional dental cleaning to remove it.
The longer that plaque and tartar remain on your teeth, the more damage they can do. Initially, they may simply irritate and inflame the gingiva, the part of your gum around the base of your teeth. This is called gingivitis, the mildest form of periodontal disease.
Ongoing inflammation eventually causes pockets to develop between your gums and teeth that fill with plaque, tartar and bacteria. Bacteria deposit endotoxin — a byproduct of their own metabolism — which is responsible for much of the inflammation around teeth. In time, these pockets become deeper and more bacteria accumulate, eventually advancing under your gum tissue. These deep infections cause a loss of tissue and bone. If too much bone is destroyed, you may lose one or more teeth.