Etiology are the causes of diseases. For occurrence of the disease to the combined effect of the main factors that cause, and the range of conditions of external and internal environment in which the factor (aetiological) exerts its action. Etiological factors may be pathogenic microorganisms, toxins, trauma, exposure, emotional impact, and many other physical, chemical and biological agents. Conditions of occurrence of the disease are a variety of external influence (cooling, fatigue, nutrition, social and geographical environment, and so on) and characteristics of the organism (under age, genotype, functional state of different organs and systems, and so on), which makes its reactivity (see Reactivity of the organism). Sometimes the same factors in some cases are etiological and others behind. So, for example, hypothermia is the cause frostbite or freezing; on the other hand, it creates favorable conditions for the emergence of a number of infectious diseases, commonly referred to as cold (sore throat, influenza, and so on). Therefore, more broadly causality should be understood as the relationship of individual factors and conditioned them to each other.
In the clinic there are mono - and politologichni diseases, depending on, whether they are caused always one or various reasons. The majority of infectious diseases is an example monoacylglycerol disease, while heart disease - polyetiological, as it can be formed in the result of rheumatic fever, syphilis, congenital defects and other reasons.
However, a causal factor even when polyetiological the disease often determines the specifics of the disease, and therefore pathogenetic therapy of the disease. So, for, the severity, contagiousness and forecast when staphylococcal and the anthrax anthrax is sharply different. Likewise has its own specifics inherent to various forms of hypertension (induced neurogenic or renal factors), intestinal obstruction (arising from the strangulation of the bowel from the outside or plugging it from the inside), and other diseases.
One etiological factor often not enough to cause the disease; need the presence of the conditions under which it can exert its influence. So, Streptococcus, usually present in the oral cavity in the form of saprophyte, cooling of the body causes sore throat due to the weakening of protective mechanisms. Even such virulent bacteria, as bruchkomitsky or diphtheria bacilli may remain in the body without causing the disease, but these same bacteria becomes pathogenic when any contributing factors (starvation, exhaustion, concomitant disease, and so on).
Causal factor after applying on the body of our knowledge, as in trauma, burn, mental shock, or a long as infections, starvation, and so on)causes changes in the different systems of the body, causing disease, in many cases, which is largely a consequence of these changes than the impact of etiological agent. The mechanism of occurrence and development of the disease, its pathogenesis (see), depend on the properties of the causative factor and reactions of the body.
The study of the etiology of each disease is absolutely necessary to conduct specific, "etiological" therapy, which aims at addressing the causes of it. Obviously, in the latter case, therapy more effective. Not less important and etiological focus of prevention, since it eliminates pathogenic causes and contributing to the disease factors to their effect on the organism (for example, the destruction of mosquitoes in malaria areas, injury prevention, identification of baillonella and so on).

Etiology (gr. aitiologia, from aitia - cause and logos - doctrine - the doctrine of the reasons and specific conditions of disease.
The problem etiology is not only medical, but also an important philosophical problem concerning the category of causality. The idea that the disease is due to effects on the body of a specific event, consistent with the provisions of the doctrine of determination (conditionality) phenomena, events in philosophy. This idea is reflected in the doctrine of etiology.
Monocausality, or mechanistic determinism, recognizing causation only form of determination and denying other types of conditionality, manifested in medicine exaggeration of the role of external pathogens and ignoring the terms of the internal environment of the organism in which these factors operate. These views are especially established with the discovery of pathogens infestation, invasion of which was recognized the reason for the emergence of infectious diseases. However, monocausality in medicine came into conflict with the new facts. Detection of cases of baillonella questioned the role of the microbe as the only cause of the disease. It became apparent that the mere introduction of the pathogen are few; there is a set of conditions - violations of protective adoptive reactions of organism in which its interaction with pathogenic agent was to become in the form of a pathological process, i.e. the infection would go into the disease.
Classic experiments Pasteur and Joubert (L. Pasteur, J. Joubert, 1877) showed that usually immune to infection with anthrax culture sticks chicken gets sick, if lower the temperature of its body. Subsequent numerous experiments and clinical studies have confirmed the need for adequate conditions for the development of infection. These facts by some researchers to have been mistakenly interpreted in the sense that only the totality of conditions of occurrence of diseases, and the reason, in particular microbe, does not play a significant role.
Mixing etiology to the notion about the conditions that determine the occurrence of pathological processes that took shape in the subjective idealist for in pathology - conditionalism (from lat. conditio - condition). The main exponent of the ideas of conditionality Fervor (M Verworn, 1912) wrote that "the concept of the reasons is a mystical concept originating from the primitive phase of human thought".
Very close to those submissions were joined to the views of constitutionalists, who believed that the disease had a pre-existence in the body and manifests itself under the influence of external influences. The reason, therefore, was given the role of incitement, which could be any external agent. Chandler (J. Tandler, 1913) formulated the basic position of constitutionalism: "the Constitution is the somatic the fate of the body".
The correct solution to the problem of causality in the etiology can be found only from the position of dialectical materialism. A prerequisite for scientific solution of the issues is the recognition of the objective nature of causation and its evaluation in the light of nedeterminata - philosophical doctrines, recognizing the interdependence of phenomena not only external, but also other types of determination - functional, structural and other properties of the object, which is the external factor.

The causal relationship in medicine subject to the law of universal communication of things and phenomena in the world. The reason is always necessary for the action, but not always in itself sufficient, since biological phenomena active not only cause (external agent), but the object to which directed its action.
The causes of diseases can be a variety of factors: chemical, physical, mechanical, biological, psychological, social and other Pre-combination of itself is harmless agent with pathogenic factor can cause disease in pathological mechanism of the conditioned reflex. Thus, for example, various external factors often become a cause of strokes. Described cases of iatrogenic diseases, when the cause of the disease was poorly spoken doctor word.
In medicine firmly established notions about the reasons "predisposing" and "producing". The first coincides with the notion of "internal environment". Producing the reasons depending on the sequence of their actions in time divided into potentially producing and obligatorily produce. For the manifestation of the first steps necessary complex of predisposing moments. It is known that in healthy people very often found virulent Mycobacterium tuberculosis, however, become ill with TB not all of their speakers. Only if you have a complex series of unfavorable external and internal predisposing factors, sharply reduces the body's resistance to infection tuberculosis mycobacteria ("potentially giving the reason") may enter the disease is tuberculosis. Similar situation takes place under the action of the etiological factor of rheumatism - beta-haemolytic Streptococcus group A. From the mass of people who have had chronic streptococcal infection (sore throat, tonsillitis), only about 3% falls ill with rheumatism.
Thus, the effect of predisposing and producing reasons may not coincide in time, and then the possibility of the disease is not implemented in the pathological process. In the role of a random kind of catalyst that facilitates the matching action predisposing and produces reasons, reinforcing their joint effect and contributing to the time of onset, can serve a variety of factors determined by the concept of "reason". The reason can be defined as "support the cause. So, for example, hematogenous osteomyelitis may occur in young people under the influence of a complex of reasons: production (the presence in the blood of pyogenic bacteria), predisposing (characteristic design of the youth bones, sensitisation to pyogenic microorganisms) and Pro (about) the injury or cooling.
There are, however, such etiologic factors that may in the short term to cause pathogenic effects without interacting with any conditions ("obligatory producing cause"). Such are the reasons for various traumatic injuries, burns and radiation injuries, etc.
In all cases, the nature of arising of pathological process will largely depend on the properties of the object, which directed the action of one or another "obligatory producing cause."
The causal factor is always in cooperation with the set of conditions - as the external inherent in the environment, and (especially) internal inherent in the body. Under the conditions of the internal environment of an organism refers to the functional condition of key physiological systems, primarily the nervous and endocrine and characteristic features of the processes at the cellular and subcellular level in organs and tissues, which ultimately formed the reactivity of the organism (see).
The enormous significance of this are the innate properties of the organism - its genotype, and age and gender.
External conditions can facilitate the operation of the causative factor, but they can make impossible the implementation of its pathogenic effect. Favorable conditions increase the body's resistance (good nutrition, regular alternation of work and rest, material security, and others), adverse - weaken it, contribute to the development of the disease (malnutrition, fatigue, and so on).
The need for cooperation between the etiological factor of disease with specific conditions, leading to disturbance of physiological functions of the body and the emergence of clinically significant disease is demonstrated by the so-called hereditary diseases. Genetic determination manifests itself through certain biochemical factors and the presence of inherited pathological mutant genes localized to chromosome apparatus the nucleus of the cell, not always lead to the development of the disease, as genetic deficiencies are amenable to correction through physiological mechanisms.
For example, based on severe forms of mental retardation, are observed with phenylketonuria, is a violation of the currency due to lack converts phenylalanine to tyrosine enzyme, controlled recessive gene. In order physiological compensate for this genetic flaw appoint a special diet, devoid of phenylalanine.
The etiology is closely associated with the pathogenesis (see), as it examines the factors that cause the occurrence of those events, the nature, mechanism and dynamics are related to pathogenesis. Even the etiology of specific infectious diseases cannot be reduced only to the action of one of the pathogenic agent.
Various non-specific "additional" stimulus (cooling, fatigue, psychological trauma, and so on), changing physiological reactivity, "pave the path to the specific causative agent, facilitating the transition of infection to disease. Of course, the qualitative features of the pathogenic agent is largely leave their mark on the specifics of the pathological process, mainly on the character of the morphological lesions. So, the specific features of the structural changes in organs and tissues, characterizing tuberculosis, diphtheria, syphilis, mostly depend on the properties of the pathogen. On the other hand, the effect of external causal factors is not always unequivocal, as is converted, transformed reactive properties of the organism. Therefore, the same effect can be caused by different reasons. So there are, for example, chronic gastritis (under the influence of different factors that cause nonspecific inflammation of the mucous membrane of stomach), bronchopneumonia, massive necrosis of the liver.
Dynamics of the disease also may not be directly connected with the specific etiological factor. The clinical picture of the disease, its course and outcome largely determined the leading physiological characteristics of the organism, its reactivity at every moment.
In some cases etiologic factor causing the disease, is not involved in its further development (for example, in the case of burn), in others it continues to operate, including the pathogenetic mechanisms of the process.
A. D. Speransky was allocated a special kind of pathological processes in which the etiological factor plays the role of a releaser (causing some changes in the nervous system, it is losing its leading position). The subsequent dynamics of the disease is determined by endogenous development neurodystrophic lesions of tissues and organs, which he described under the name of "standard form of nerve degeneration", and the influence of the causal factor is modified so that its specificity partially or completely lost.
In this respect, of great interest to study the role of the so-called autoimmunological or autoimmune reactions in a variety of somatic and infectious and allergic diseases.
Presents evidence that the reaction of autoallergy is an important intermediate etiological link, which puts its mark on the specifics of many diseases. Thus, the post-vaccination encephalomyelitis are the clinical expression of special "reactions of the second order" (A. D. ADO). The latter are caused by the formation in the body "intermediate" autoantigens due to the interaction of nairovirus with nervous tissue and production of autoantibodies against these antigens.
Eventually, reactions "second order" are characterized by damage to the nervous tissue, which determines ultimately the specificity of clinical disease.
There is a relatively large group of the so-called polietiologic disease in which external causes diseases can perform a variety of agents, including but apparently common pathogenetic mechanisms. Such, for example, allergic diseases, bronchial asthma, eczema.
The study of the etiology is not only important theoretical and practical value. Modern achievements in the field of specific diagnostics, serotherapy and prevention of infectious diseases would have been impossible without deep studies in the field of etiology. On this basis developed specific principles ("etiological) therapy aimed at neutralization of pathogenic influence of the causal factors of the disease or its removal from the body. These principles, in particular, guided by surgeons in primary treatment of wounds and treatment of wound infections.
Elimination of the etiological factor at all stages of the disease beneficial for the body.