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.
Ibn Zuhr (Avenzoar) was the first physician to provide a scientific etiology for the inflammatory diseases of the ear, and the first to clearly discuss the causes of stridor. Through his dissections, he proved that the skin disease scabies was caused by a parasite, a discovery which upset the Galenic theory of humorism, and he was able to successfully remove the parasite from a patient's body without any purging or bleeding.
When the Black Death bubonic plague reached al-Andalus in the 14th century, Ibn Khatima posited that infectious diseases are caused by microorganisms which enter the human body. Another Andalusian physician, Ibn al-Khatib (1313-1374), wrote a treatise called On the Plague, stating that contagion can spread through garments, vessels and earrings.
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, one etiology, such as Epstein-Barr virus, may in different circumstances produce different diseases, such as mononucleosis, or nasopharyngeal carcinoma, or Burkitt's lymphoma.