In the early stage of gingivitis, bacteria in plaque build up, causing the gums to become inflamed and to easily bleed during tooth brushing.
Although the gums may be irritated, the teeth are still firmly planted in their sockets. No irreversible bone or other tissue damage has occurred at this stage.
When gingivitis is left untreated, it can advance to periodontitis. In a person with periodontitis, the inner layer of the gum and bone pull away from the teeth and form pockets.