(from the Greek aitia – cause and logos – teaching) – the cause of the occurrence of a disease or pathological condition. The cause of the disease should be considered a factor without which it cannot occur under any conditions. In a number of cases, the same disorder occurs under the influence of factors of different nature; In this case, we speak of polyetiology, i.e. it can be caused by various reasons. As factors of the external and internal environment that can cause a disease or disorder, there are physical (trauma, thermal exposure, ionizing radiation, atmospheric influences, etc.), chemical (acids, alkalis, poisons, etc.), biological (pathogenic microbes, viruses, various endogenous substances) and social. The disease can also be caused by a deficiency in the environment or in the body of substances necessary for life. The development of the disease is determined not only by the presence of a pathogenic factor, an important role is played by the individual characteristics of the body (its reactivity, constitution, heredity, age, gender, etc.) and social factors (exhausting work, malnutrition, unsanitary living conditions, alcoholism, etc.). Etiotropic treatment is aimed at eliminating the cause of the disease (for example, antibacterial treatment for infectious diseases). Pathogenetic treatment is aimed at the processes that occur during the development of the disease. Symptomatic treatment is aimed at individual manifestations (symptoms) of the disease.