Your bone and gum tissue should fit snugly around your teeth like a turtleneck around your neck.
When you have periodontal disease, this supporting tissue and bone is destroyed, forming "pockets" around the teeth.
Over time, these pockets become deeper, providing a larger space for bacteria to live. As bacteria develop around the teeth, they can accumulate and advance under the gum tissue.
These deep pockets collect even more bacteria, resulting in further bone and tissue loss. Eventually, if too much bone is lost, the teeth will need to be extracted.