Periodontitis is a set of inflammatory diseases affecting the periodontium, i.e., the tissues that surround and support the teeth. Periodontitis involves progressive loss of the alveolar bone around the teeth, and if left untreated, can lead to the loosening and subsequent loss of teeth. Periodontitis is caused by microorganisms that adhere to and grow on the tooth's surfaces, along with an overly aggressive immune response against these microorganisms. A diagnosis of periodontitis is established by inspecting the soft gum tissues around the teeth with a probe (i.e., a clinical examination) and by evaluating the patient's X-ray films (i.e. a radiographic examination), to determine the amount of bone loss around the teeth.[1] Specialists in the treatment of periodontitis are periodontists; their field is known as "periodontology" or "periodontics".

The 'severity' of disease refers to the amount of periodontal ligament fibers that have been lost, termed 'clinical attachment loss'. According to the American Academy of Periodontology, the classification of severity is as follows:[3]
Mild: 1–2 mm (0.039–0.079 in) of attachment loss
Moderate: 3–4 mm (0.12–0.16 in) of attachment loss
Severe: ≥ 5 mm (0.20 in) of attachment loss

Signs and symptoms

In the early stages, periodontitis has very few symptoms; and in many individuals the disease has progressed significantly before they seek treatment.
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 occur even in gingivitis, where there is no attachment loss)
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 tooth brush.)
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)
Patients should realize 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 patient.