Periodontitis is a type of gum disease, which means it affects the gums. It starts when bacteria that naturally live in your mouth makes your gums red, swollen, and sore. When left untreated, the bacteria can cause your gums to pull away from your teeth and form pockets where more bacteria can grow. This can damage the tissue and bones that support your teeth and eventually cause some of your teeth to fall out. Taking care of your mouth, like brushing and flossing, helps get rid of the bacteria and can help prevent periodontitis.