There is lack of consensus among investigators concerning the etiology of Alzheimer's disease. Clues are not lacking, however, and we have assessed them in a broad biologic context. This inquiry has led us to regard Alzheimer's disease as a multifactorial disorder in which a putative infective agent is an essential element. Despite seeming competition among current hypotheses, there is overall unity. The concept that Down's syndrome is a congenital form of Alzheimer's disease and that both conditions are the result of a ubiquitous infective pathogen that affects genetically susceptible individuals offers the broadest unification. In both conditions slow infection develops against the background of aging. Indirect evidence involving immunologic and other biologic phenomena supports the postulated infectious origin. Overlapping pathologic and clinical features of Alzheimer's disease and the known transmissible encephalopathies suggest a similar pathogenesis.