[Principles and definitions in epidemiology (author's transl)]. 
When medical research became concerned with the aetiology of chronic diseases, it also became necessary to adapt the principles enounced by Koch for the study of acute infectious diseases and this was expressed most satisfactorily by Evans.  These changes entailed the profusion of new definitions of epidemiology which were recently reviewed by Evans and led him to propose a new one--the 24th.  One important characteristic of epidemiology, however, does not appear clearly in these various definitions, namely that it is a branch of medicine.  In short, it could be defined as the medical science of human groups.  The authors show how the work of the epidemiologist parallels that of the medical practitioner facing a patient.  The observation of the distribution of the disease within the population, referred to as descriptive epidemiology corresponds to the clinical examination and the enumeration of symptoms in the individual.  The clinical interpretation of the symptoms and reasoning leading to a diagnosis is paralleled by the work of the "analytical" epidemiologist trying to discover the aetiological factors of the disease.  The clinician finally takes action and administers treatment, while the findings of the epidemiologist find their application in Public Health preventive action.  The authors conclude by proposing a modified version of Evans' definition of epidemiology; it reads: "Epidemiology is the quantitative study of physiological, prepathological and pathological conditions and processes in human populations, in relation to natural or man-made environmental factors which account for or favour their inception, development or disappearance.  Its purpose is to prevent health disorders, to reduce their incidence or to lessen their ill effects.  Epidemiology uses the resources of all medical sciences, plus all those--including statistics--imposed by its specific subject.  It is the medical science of human groups.  It completes and expands traditional medicine dealing with individuals".