We found a great article on colgate.com about Periodontitis:
Periodontitis is caused by bacteria that attach to the teeth at the gum line and result in an infection. Your body tries to fight the infection. Some of the substances that your body produces can be harmful. They damage the jaw bone and the ligament around the teeth.
If this process continues, the teeth become loose. Pockets form between the teeth and gums. These pockets are places where more bacteria grow. If your periodontitis is not treated, your teeth can become loose and even fall out.”
You can read the rest of the article here: Gum Disease-What is Periodontitis?