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aetiologythe study of causation, especially of diseases, social pathologies, etc. (e.g. Durkheim's Suicide, 1897).
the cause of disease; a branch of pathology that studies causality in medicine. Specific etiological factors are known for a number of diseases, for example, most infectious and parasitic diseases. The causative agent of tuberculosis is known definitively to be Mycobacterium ; that of tertian malaria is Plasmodium vivax. These microorganisms never cause diseases other than tuberculosis and malaria, and these diseases, in turn, cannot be caused by other pathogens. We speak in such cases of specific etiology in order to emphasize the unambiguous cause-and-effect relationship existing between the etiological factor and the disease.
An example of etiological relations of a different kind is Staphylococcus albus, which causes not only septic endocarditis but other diseases, for example, furuncle or pneumonia. Protrac-tic septic endocarditis may be caused by a variety of microbes, including Streptococcus viridans. This type of etiological relationship is called nonspecific. The presence of the etiological factor is usually necessary but not always sufficient for the disease to develop. The organism has to be somewhat susceptible to the pathogenic principle, and the total inevitability of disease under natural conditions is rare.
The pathogenicity of causative agents varies widely. It is very high in plague, cholera, smallpox, and measles. However, only a few persons develop tonsillitis in a “streptococcal environment.” Mycobacterium tuberculosis, the undisputed cause of tuberculosis, is present in the great majority of human beings, but only a few contract the disease. Its development, location, and severity are largely determined by individual reactivity. Thus, an infectious disease results from the interaction of a microbe and the human organism. Viruses, bacteria, and other living organisms are not the only definitively known causes of disease. Physical (including radiation) and chemical agents also cause injury.
The etiological role of the hereditary factors present in the organism at the cellular and molecular levels is incomparably more complex. Distortion of the genetic information underlying hereditary diseases can also be related justifiably to etiology and to the mechanism of development of disease (that is, pathogenesis), because the strictly etiological role belongs to those factors, often still obscure, that caused a mutation, for example, in the patient’s parents. Disease in individuals with a hereditary predisposition is interpreted differently, because such a predisposition creates only the conditions conducive to the development of disease. The disease is regarded in the chain of cause-and-effect relations as a pathogenetic rather than etiological factor.
With respect to an actual disease in a particular individual, the etiology is always related to a number of conditions either resisting or promoting the development of the disease. Therefore, the etiology may be considered separately from the pathogenesis only in connection with an analysis of it and not in an actual therapeutic and diagnostic situation. It is often difficult to distinguish the original cause of a disease among the many factors involved. The etiology of some human diseases has not been established, although the pathogenesis of several of them has been thoroughly studied and the diseases are regarded as independent nosologic entities. However, this cannot serve as a basis for denying the very existence of the original cause or for maintaining that there are many causes, as in chronic diseases whose etiology has not yet been discovered (tumors, peptic ulcer, atherosclerosis). Since the term “etiology” is usually applied only to the main cause of a disease and not to the mechanism of its development, symptoms, complications, and sequelae, we can speak, for example, of the allergic nature of some lung diseases but not of their allergic etiology.
It is of considerable practical importance to discover the etiology of a disease, because it pro vides a sound basis not only for pathogenetic and expectant treatment but also for specific action by therapeutic agents on the causative factor, for example, the use of antibiotics to treat infectious diseases. Determination of the etiology of a disease is also the basis for rational preventive measures (for example, vaccination against infectious diseases).
The term “etiology” is also used in veterinary medicine.
I. V. MARTYNOV