In medicine, the term refers to the causes of diseases or pathologies. Where no etiology can be ascertained, the disorder is said to be idiopathic. Traditional accounts of the causes of disease may point to the "evil eye". The Ancient Roman scholar Marcus Terentius Varro put forward early ideas about microorganisms in a 1st-century BC book titled On Agriculture.
Medieval thinking on the etiology of disease showed the influence of Galen and of Hippocrates. Medieval European doctors generally held the view that disease was related to the air and adopted a miasmatic approach to disease etiology.
Etiological discovery in medicine has a history in Robert Koch's demonstration that the tubercle bacillus (Mycobacterium tuberculosis complex) causes the disease tuberculosis, Bacillus anthracis causes anthrax, and Vibrio cholerae causes cholera. This line of thinking and evidence is summarized in Koch's postulates. But proof of causation in infectious diseases is limited to individual cases that provide experimental evidence of etiology.
In epidemiology, several lines of evidence together are required to infer causation. Sir Austin Bradford-Hill demonstrated a causal relationship between smoking and lung cancer, and summarized the line of reasoning in the epidemiological criteria for causation. Dr. Al Evans, a US epidemiologist, synthesized his predecessors' ideas in proposing the Unified Concept of Causation.
Further thinking in epidemiology was required to distinguish causation from association or statistical correlation. Events may occur together simply due to chance, bias or confounding, instead of one event being caused by the other. It is also important to know which event is the cause. Careful sampling and measurement are more important than sophisticated statistical analysis to determine causation. Experimental evidence involving interventions (providing or removing the supposed cause) gives the most compelling evidence of etiology.
Etiology is sometimes a part of a chain of causation. An etiological agent of disease may require an independent co-factor, and be subject to a promoter (increases expression) to cause disease. An example of all the above, which was recognized late, is that peptic ulcer disease may be induced by stress, requires the presence of acid secretion in the stomach, and has primary etiology in Helicobacter pylori infection. Many chronic diseases of unknown cause may be studied in this framework to explain multiple epidemiological associations or risk factors which may or may not be causally related, and to seek the actual etiology.
Some diseases, such as diabetes or hepatitis, are syndromically defined by their signs and symptoms, but include different conditions with different etiologies. Conversely, a single etiology, such as Epstein-Barr virus, may in different circumstances produce different diseases such as mononucleosis, nasopharyngeal carcinoma, or Burkitt's lymphoma.
See also 
- "Aetiology". Oxford English Dictionary (2nd ed. ed.) (Oxford University Press). 2002. ISBN 0-19-521942-2.
- Greene J (1996). "The three C's of etiology". Wide Smiles. Archived from the original on 2007-06-30. Retrieved 2007-08-20. Discusses several examples of the medical usage of the term etiology in the context of cleft lips and explains methods used to study causation.
- Meleis, Afaf Ibrahim (June 1981). "The Arab American in the health care system". American Journal of Nursing (PDF ) 81 (06): 1180–1183. "While germ theory is not refuted, it does exist side by side with other disease etiologies. The evil eye (al hassad or al ain al Weh- sha) is one causative agent for the Arab."
- Varro On Agriculture 1,xii Loeb
- Maimonides: an early but accurate view on the treatment of hemorrhoids -- Magrill and Sekaran 83 (979): 352 -- Postgraduate Medical Journal
- Case study: the history and ethics clean air
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