As of 2017 it was not certain what species were most responsible for causing harm, but gram-negative anaerobic bacteria, spirochetes, and viruses have been suggested; in individual people it is sometimes clear that one or more species is driving the disease. Research in 2004 indicated three gram negative anaerobic species: Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, Bacteroides forsythus and Eikenella corrodens.Plaque may be soft and uncalcified, hard and calcified, or both; for plaques that are on teeth the calcium comes from saliva; for plaques below the gumline, it comes from blood via oozing of inflamed gums.The damage to teeth and gums comes from the immune system as it attempts to destroy the microbes that are disrupting the normal symbiosis between the oral tissues and the oral microbe community. As in other tissues, Langerhans cells in the epithelium take up antigens from the microbes, and present them to the immune system, leading to movement of white blood cells into the affected tissues. This process in turn activates osteoclasts which begin to destroy bone, and it activates matrix metalloproteinases that destroy ligaments. So, in summary, it is bacteria which initiates the disease, but key destructive events are brought about by the exaggerated response from the hosts immune system. Classification
There were several attempts to introduce an agreed-upon classification system for periodontal diseases: in 1989, 1993, 1999, and 2017. 1999 classification
The 1999 classification system for periodontal diseases and conditions listed seven major categories of periodontal diseases, of which 2–6 are termed destructive periodontal disease, because the damage is essentially irreversible.
Symptoms may include:

Redness or bleeding of gums while brushing teeth, using dental floss or biting into hard food (e.g., apples) (though this may also occur in gingivitis, where there is no attachment loss gum disease )
Gum swelling that recurs
Spitting out blood after brushing teeth
Halitosis, or bad breath, and a persistent metallic taste in the mouth
Gingival recession, resulting in apparent lengthening of teeth (this may also be caused by heavy-handed brushing or with a stiff toothbrush)
Deep pockets between the teeth and the gums (pockets are sites where the attachment has been gradually destroyed by collagen-destroying enzymes, known as collagenases)
Loose teeth, in the later stages (though this may occur for other reasons, as well)Gingival inflammation and bone destruction are largely painless. Hence, people may wrongly assume painless bleeding after teeth cleaning is insignificant, although this may be a symptom of progressing periodontitis in that person. Associated conditions
Periodontitis has been linked to increased inflammation in the body, such as indicated by raised levels of C-reactive protein and interleukin-6. It is associated with an increased risk of stroke, myocardial infarction, atherosclerosis and hypertension. It also linked in those over 60 years of age to impairments in delayed memory and calculation abilities. Individuals with impaired fasting glucose and diabetes mellitus have higher degrees of periodontal inflammation, and often have difficulties with balancing their blood glucose level owing to the constant systemic inflammatory state, caused by the periodontal inflammation.