Contemporary hypotheses that consider the severe forms of periodontal disease to be infections caused by site-specific microbes fail to satisfactorily explain the epidemiologic, anthropologic, and clinical features of periodontal diseases. The microbes that have been designated as periodontopathogens are commensal bacteria present in periodontal health and disease. Association of specific bacteria with various disease forms has been established, but association is confused with causation. None of the periodontal diseases can be transmitted between individuals or between diseased and healthy sites of a susceptible person. Past and present concepts of the etiology of the periodontal diseases are reviewed, and the deficiencies of contemporary periodontal theory are outlined. Host factors rather than bacteria determine whether gingivitis extends to horizontal periodontitis. Angular alveolar lesions, the severe form of periodontal bone loss, are hypothesized to be caused by the spread of pulpal inflammation to the adjacent periodontal tissues. When the resultant dental abscess becomes contiguous with the alveolar crest and gingival sulcus, secondary colonization of deep pockets by site-specific oral bacteria-selected for by the complex conditions of the site-can occur. This explanation accounts for the distribution of periodontopathogens, the localization of angular alveolar lesions, and the bursts of activity by which the disease progresses.