It may affect a variable number of teeth and it has variable rates of progression. Chronic periodontitis is initiated and sustained by bacterial plaque, but host defense mechanisms play an integral role in its pathogenesis. The progressive nature of the disease can only be confirmed by repeated examinations. It is reasonable to assume that the disease will progress further if treatment is not provided. Chronic periodontitis can be further characterized by extent and severity. Extent is the number of sites involved and can be described as localized or generalized. As a general guide, extent can be characterized as localized if ≤30% of the sites are affected and generalized if >30% of the sites are affected. Severity can be described for the entire dentition or for individual teeth and sites. As a general guide, severity can be categorized on the basis of the amount of clinical attachment loss (CAL) as follows: Slight = 1 to 2 mm CAL, Moderate = 3 to 4 mm CAL, and Severe = ≥5 mm CAL. The group acknowledged that in clinical practice, there are recurrent and refractory (non-responsive) cases of periodontitis. However, the group did not believe that there is enough evidence to place these cases in their own disease categories since any type of periodontitis can recur and a small percentage of cases can be non-responsive to therapy. The group concluded that recurrent periodontitis represents a return of periodontitis and is not a separate disease entity. For a variety of identifiable and non-identifiable reasons, not all cases of periodontitis have a successful treatment outcome. Such cases can be referred to as refractory periodontitis but do not necessarily constitute a separate disease entity. More research is needed to identify the reasons for the failure to respond to treatment. Since conditions or factors that modify the gingiva in gingivitis (see Gingival Diseases, pages 7-31) also modify the gingiva in periodontitis, they were not considered in this section of the classification.