Etiology

Overview
Etiology (alternately aetiology, aitiology) is the study of causation. Derived from the Greek αίτιολογία, "giving a reason for" (αἰτία "cause" + -logy).

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, and medicine, 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. Etiological discovery in medicine has a history in Robert Koch's demonstration that the tubercle bacillus (Mycobacterium tuberculosis complex) causes the disease tuberculosis, that Bacillus anthracis causes anthrax, and that cholera is caused by Vibrio cholerae. This line of thinking and evidence is summarized in Koch's postulates. Proof of causation in infectious diseases is limited, however, to individual cases that provide experimental evidence of etiology.

In epidemiology, several lines of evidence taken in aggregate are required to infer causation. Sir Adrian 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, put forward the Unified Concept of Causation, a synthesis of the predecessors' ideas.

Etiological research in medicine has required further thinking in epidemiology -  we may distinguish what seen to be associated or statistically correlated, as due to several possible relationships. Things may be associated in observation due to chance, or due to bias or confounding, as well as due to causation (or reverse causation). Careful sampling and measurement are more important in teasing out causation from chance, bias or confounding than sophisticated statistical analysis. Experimental evidence, involving interventions (providing or removing the supposed cause) gives the most compelling evidence of etiology.

Thus etiology may be one part of a chain of causation. An etiological agent (sine qua non) of disease may require an independent co-factor (necessary but not sufficient), and be subject to a promoter (increases expression) in producing a disease. An example of all the above would be the late recognition that peptic ulcer disease may be induced by stress, requires the presence of acid secretion in the stomach, and have 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, are syndromically defined by their signs and symptoms, but include more than one condition, and therefore can have more than one etiology. Alternatively, one etiology of disease such as Epstein-Barr virus may produce more than one disease, such as mononucleosis, or nasopharyngeal carcinoma, or Burkitt's lymphoma given different circumstances.

Mythology
An etiological 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 delphys ("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 Julus. Other examples of etiological myth come from the Bible, such as the setting of the rainbow in the heavens as a sign of God's covenant with Noah (Genesis 9); or the story of Lot's wife in Genesis 19 (specifically 26), which explains why there are pillars of salt in the area of the Dead Sea. 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.