The role of epidemiology in the study of periodontal diseases.

Models of disease are collections of theory on its etiology and progression, based on research as far as possible but often including some untested hypotheses. As such, these models have to be modified from time to time as new research findings come out and sometimes have to be revised extensively. This is the case with the models for periodontal diseases. In fact, knowledge of the disease process has expanded so rapidlv that it is easy to forget that, as recently as the mid-l960s, a widely accepted model of periodontal disease included the following: