There are three stages of periodontal disease: gingivitis, periodontitis, and advanced periodontitis.
More Info: Nearly seventy-five percent of American adults suffer from some form of gum disease.  The American Academy of Periodontology estimates that fifty percent of Americans over thirty have the most advanced form of the disease. 
What Is Periodontal Disease?
Periodontal disease or gum disease caused by the bacteria found in plaque. Plaque is a sticky film of bacteria that forms on your teeth. When plaque is not removed, the gums, or the gingivae, start to pull away from your teeth, which in turn causes bacteria-filled pockets to form in the gums. This leads to the inflammation of the gums, which can progress into a more serious disease affecting the bones that support the teeth, if left untreated. That is why it is important to detect gum disease at its earliest stage, which is gingivitis, to prevent it from progressing and wreaking sometimes irreversible damage on your teeth. 
First Stage: Gingivitis
The first stage of gum disease is gingivitis. The presence of gingivitis is detected by the inflammation of the gums along the gum line. This is caused by the buildup of plaque, which as discussed above, causes the gums the gums to pull away from the teeth and therefore causes pockets of bacteria to form along the gum line. Another telltale sign of gingivitis is the bleeding of the gums when you floss or brush your teeth. 
Second Stage: Periodontitis
Periodontitis a more advanced stage of gingivitis, wherein the bacteria caused by the plaque build-up on your teeth, has already spread to the bones and tissue fibers that support your teeth. It is in this stage that the said bones and fibers are irreversibly damaged. Here, the pockets of bacteria increase and the infection is more wide spread. 
Third Stage: Advanced Periodontitis
When periodontitis worsens into advanced periodontitis, the fibers and bones that support your teeth are destroyed, which is why the teeth they support become loose and may have to be removed. 
 University of Maryland Medical Center
Periodontal Disease Risk Factors
 American Academy of Periodontology
Periodontal Disease Fact Sheet
 Journal of American Dental Association
What Is Gum Disease?”