Etiology

Etiology (alternatively aetiology, aitiology ) is the study of causation, or origination. The word is derived from the Greek αἰτιολογία, aitiologia, "giving a reason for" (αἰτία, aitia, "cause"; and -λογία, -logia).

The word is most commonly used in medical and philosophical theories, where it is used to refer to the study of why things occur, or even the reasons behind the way that things act, and is used in philosophy, physics, psychology, government, medicine, theology and biology in reference to the causes of various phenomena. An etiological myth is a myth intended to explain a name or create a mythic history for a place or family.

Medicine
In medicine in particular, the term refers to the causes of diseases or pathologies. 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.

Mythology
An etiological myth, or origin myth, is a myth intended to explain the origins of cult practices, natural phenomena, proper names and the like. For example, the name Delphi and its associated deity, Apollon Delphinios, are explained in the Homeric Hymn which tells of how Apollo carried Cretans over the sea in the shape of a dolphin (delphis) to make them his priests. While Delphi is actually related to the word delphus ("womb"), many etiological myths are similarly based on folk etymology (the term "Amazon", for example). In the Aeneid (published circa 17 BC), Vergil claims the descent of Augustus Caesar's Julian clan from the hero Aeneas through his son Ascanius, also called Iulus. The story of Prometheus' sacrifice-trick in Hesiod's Theogony relates how Prometheus tricked Zeus into choosing the bones and fat of the first sacrificial animal rather than the meat to justify why, after a sacrifice, the Greeks offered the bones wrapped in fat to the gods while keeping the meat for themselves.