Periodontal disease is an infection of the gums and affects the bone that stabilizes teeth. Bacteria collects in the small spaces between the gums and the teeth. When you brush your teeth, most of this bacteria is removed, but a professional dental cleaning every six months ensures this bacteria is removed.
If bacteria is allowed to accumulate to a high enough level, the body responds just like an infection anywhere else in the body: it causes inflammation, swelling, redness and can cause pain, but not always.
The bone that surrounds the tooth senses the infection in the gums and responds by receding to escape the infection. The result is bone loss around the infected teeth.
In turn, the pockets between the gums and the teeth become deeper and easier for bacteria to accumulate. This results in a deeper infection and additional recession of the bone that continues to try to escape the infection.