Etiological Factors

Exogenous and Endogenous Conditions

For an etiological factor to exert its pathogenic effect, specific conditions are necessary. By themselves, they are unable to cause disease, but they can significantly facilitate or, conversely, hinder its development. Traditionally, they are divided into two large groups:

• External (exogenous) conditions combine environmental factors. These include ecological issues (water and air pollution, exposure to industrial, agricultural, and household agents), nutritional disorders (quantitative deficiency or qualitative inadequacy of diet), schedule disruptions (imbalance between work and active rest), and the social environment generating frequent conflict situations and chronic stress.
• Internal (endogenous) conditions are the intrinsic properties of the organism itself that determine its readiness to encounter a pathogen. This includes baseline resistance to a specific factor, body build features (constitution), sex, age, and specifics of the higher nervous activity (HNA) type.

Organism Reactivity and Its Significance

With the development of society and the advancement of medicine, the range of crude pathogenic factors narrows: the level of trauma decreases, the number of severe poisonings declines, and general living conditions improve. Against this background, the role of the organism's own state—its reactivity—increases colossally in modern pathology. This is as fundamental a quality of living matter as growth, reproduction, heredity, or metabolism.

Reactivity is the property of an integral organism (necessarily possessing a nervous system) to respond differentially to stimuli of the external and internal environment by altering its vital activity. The effect of encountering an etiological factor is not always linear:

1. Obligate pathogens (extreme temperatures, severe mechanical trauma, high doses of radiation) possess absolute aggressiveness and cause pathology in all people without exception.
2. The conditional nature of factor action directly depends on resistance. For example, during a rapid ascent to high altitude, mountain sickness does not develop in everyone. Also widely known is the asymptomatic carriage of pathogenic microbes in the tonsils without developing tonsillitis, or the presence of Mycobacterium tuberculosis in the lungs without the clinical picture of tuberculosis.
3. Pathogenic action of indifferent factors manifests under altered reactivity. Completely ordinary odors or everyday food products (berries, chocolate, chicken eggs) are capable of provoking disease (allergy).

Classification of Reactivity

Reactivity is not a homogeneous concept. Depending on biological properties, three main levels are distinguished:

• Species reactivity. Formed during long-term evolution through mutations, natural selection, and hereditary fixation of crucial traits. It determines the features of a specific biological species. Classic examples: rabbits are completely unsusceptible to the syphilis agent, and they do not develop atherosclerosis, unlike humans.
• Group reactivity. Depends on an individual's belonging to a specific category within their species. Age reactivity explains why children, due to immaturity of the immune system, are more susceptible to infectious diseases. Sex reactivity determines better tolerance to blood loss in females and higher resistance to physical exertion in males. Constitutional reactivity is based on stable morphofunctional and mental features: for example, asthenics tolerate strong and prolonged loads significantly worse than normosthenics.
• Individual reactivity. Formed on the basis of inherited information, individual variability, and the unique life experience of a specific organism. Unlike species and constitutional reactivity, it can be not only physiological but also pathological.

Duration of Action of the Etiological Factor

A fundamental conclusion of general pathophysiology states: the possibility of occurrence, course features, and outcomes of pathology are determined by a combination of pathogen properties, organism reactivity, and environmental conditions. At the same time, the temporal nature of the interaction between cause and organism can proceed according to two different scenarios:

• Short-term action of the cause. The etiological factor acts exclusively as a starting trigger. It launches the pathological process, after which its presence is no longer required for the further development of the disease. Typical examples of such a mechanism include radiation sickness, frostbite, thermal burns, myocardial infarction, and tumor formation.
• Permanent presence of the cause. The etiological factor not only triggers the disease but continues to act on the organism continuously throughout the entire period of the disease. This is characteristic of conditions such as coronary artery disease (CAD), diabetes mellitus (DM), various symptomatic arterial hypertensions, and hyperthyroid states.