In medicine, etiology refers to the cause or 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 species of the pathogenic bacteria Mycobacterium tuberculosis 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 for causal inference. Austin Bradford Hill demonstrated a causal relationship between tobacco smoking and lung cancer, and summarized the line of reasoning in the Bradford Hill criteria, a group of nine principles to establish epidemiological causation. This idea of causality was later used in a proposal for a Unified concept of causation.
Disease causative agent
The infectious diseases are caused by infectious agents or pathogens. The infectious agents that cause disease fall into five groups: viruses, bacteria, fungi, protozoa, and helminths (worms).
The term can also refer to a toxin or toxic chemical that causes illness.
Chain of causation and correlation
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.
Related to this, sometimes several symptoms always appear together, or more often than what could be expected, though it is known that one cannot cause the other. These situations are called syndromes, and normally it is assumed that an underlying condition must exist that explains all the symptoms.
Other times there is not a single cause for a disease, but instead a chain of causation from an initial trigger to the development of the clinical disease. 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. These are called heterogeneous conditions.
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.
An endotype is a subtype of a condition, which is defined by a distinct functional or pathobiological mechanism. This is distinct from a phenotype, which is any observable characteristic or trait of a disease, such as morphology, development, biochemical or physiological properties, or behavior, without any implication of a mechanism. It is envisaged that patients with a specific endotype present themselves within phenotypic clusters of diseases.
Other example could be AIDS, where an HIV infection can produce several clinical stages. AIDS is defined as the clinical stage IV of the HIV infection.
- Rothman, Kenneth J.; Greenland, Sander; Poole, Charles; Lash, Timothy L. (2008). "Causation and Causal Inference". Modern Epidemiology (Third ed.). Philadelphia: Lippincott Williams & Wilkins. pp. 6–7. ISBN 978-0-7817-5564-1. https://books.google.com/books?id=Z3vjT9ALxHUC&pg=PA6.
- Aetiology (2nd ed.). Oxford University Press. 2002. ISBN 0-19-521942-2.
- Greene J (1996). "The three C's of etiology". Wide Smiles. http://www.widesmiles.org/cleftlinks/WS-364.html. 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" (PDF). American Journal of Nursing 81 (6): 1180–1183. doi:10.1097/00000446-198106000-00037. PMID 6909011. https://dx.doi.org/10.1097%2F00000446-198106000-00037. "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
- Magrill, Dan; Sekaran, Prabhu (May 1, 2007). "Maimonides: an early but accurate view on the treatment of haemorrhoids". Postgraduate Medical Journal 83 (979): 352–354. doi:10.1136/pgmj.2006.053173. PMID 17488868. https://pmj.bmj.com/content/83/979/352.
- Brimblecombe, Peter; Nicholas, Frances M. (May 19, 1993). Berry, R.J.. ed. Environmental Dilemmas: Ethics and decisions. Springer Netherlands. pp. 72–85. doi:10.1007/978-0-585-36577-0_5. https://doi.org/10.1007/978-0-585-36577-0_5.
- Evans, Alfred S.; Evans, Terry (1993) (in en). Causation and Disease: A Chronological Journey. Springer Science & Business Media. ISBN 9780306442834. https://books.google.com/books?id=KdJeGM4iS9sC&pg=PA1. Retrieved 7 December 2021.
- Lötvall, J.; Akdis, C. A.; Bacharier, L. B.; Bjermer, L.; Casale, T. B.; Custovic, A.; Lemanske, R. F. Jr.; Wardlaw, A. J. et al. (2011). "Asthma endotypes: A new approach to classification of disease entities within the asthma syndrome". J Allergy Clin Immunol 127 (2): 355–60. doi:10.1016/j.jaci.2010.11.037. PMID 21281866.
- Castro, Kenneth G.; Ward, John W.; Slutsker, Laurence; Buehler, James W.; Jaffe, Harold W.; Berkelman, Ruth L. (1993). "1993 Revised Classification System for HIV Infection and Expanded Surveillance Case Definition for AIDS among Adolescents and Adults". Clinical Infectious Diseases 17 (4): 802–810. doi:10.1093/clinids/17.4.802.