Cause, also known asetiology (/iːtiˈɒlədʒi/) andaetiology, is thereason or origination of something.[1]
The wordetiology is derived from theGreekαἰτιολογία,aitiologia, "giving a reason for" (αἰτία,aitia, "cause"; and-λογία,-logia).[2]
In medicine,etiology refers to the cause or causes ofdiseases orpathologies.[3] Where no etiology can be ascertained, the disorder is said to beidiopathic.Traditional accounts of the causes of disease may point to the "evil eye".[4]TheAncient Roman scholarMarcus Terentius Varro put forward early ideas aboutmicroorganisms in a 1st-century BC book titledOn Agriculture.[5]
Medieval thinking on the etiology of disease showed the influence ofGalen and ofHippocrates.[6] MedievalEuropean doctors generally held the view that disease was related to the air and adopted amiasmatic approach to disease etiology.[7]
Etiological discovery in medicine has a history inRobert Koch's demonstration that species of thepathogenic bacteriaMycobacterium tuberculosis causes the diseasetuberculosis;Bacillus anthracis causesanthrax, andVibrio cholerae causescholera. This line of thinking and evidence is summarized inKoch's postulates. But proof of causation in infectious diseases is limited to individual cases that provide experimental evidence of etiology.[citation needed]
Inepidemiology, several lines of evidence together are required to forcausal inference.Austin Bradford Hill demonstrated a causal relationship betweentobacco smoking andlung cancer, and summarized the line of reasoning in theBradford Hill criteria, a group of nine principles to establish epidemiological causation. This idea of causality was later used in a proposal for aUnified concept of causation.[8]
The infectious diseases are caused by infectious agents orpathogens. The infectious agents that cause disease fall into five groups: viruses, bacteria, fungi, protozoa, and helminths (worms).[9]
The term can also refer to a toxin or toxic chemical that causes illness.
Further thinking in epidemiology was required todistinguish causation from association or statistical correlation. Events may occur together simply due tochance,bias orconfounding, 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.[citation needed]
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 calledsyndromes, and normally it is assumed that an underlying condition must exist that explains all the symptoms.[citation needed]
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 thatpeptic ulcer disease may be induced by stress, requires the presence of acid secretion in the stomach, and has primary etiology inHelicobacter pylori infection. Many chronic diseases of unknown cause may be studied in this framework to explain multiple epidemiological associations orrisk factors which may or may not be causally related, and to seek the actual etiology.
Some diseases, such asdiabetes orhepatitis, are syndromically defined by theirsigns and symptoms, but include different conditions with different etiologies. These are calledheterogeneous conditions.[citation needed]
Conversely, a single etiology, such asEpstein–Barr virus, may in different circumstances produce different diseases such asmononucleosis,nasopharyngeal carcinoma, orBurkitt's lymphoma.[citation needed]
Anendotype is a subtype of a condition, which is defined by a distinct functional orpathobiological mechanism. This is distinct from aphenotype, which is any observable characteristic or trait of adisease, such asmorphology, development, biochemical or physiological properties, or behavior, without any implication of a mechanism. It is envisaged thatpatients with a specific endotype present themselves within phenotypic clusters of diseases.
One example is asthma, which is considered to be asyndrome, consisting of a series of endotypes.[10] This is related to the concept ofdisease entity.
Other example could beAIDS, where an HIV infection can produce several clinical stages. AIDS is defined as theclinical stage IV of the HIV infection.[11]
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.
The dictionary definition ofetiology at Wiktionary