Due to the prevalence and diversity of both symptoms and outcomes of COVID-19, understanding the factors that determine the risk of the severe course of this disease and its possible complications is of particular importance. In this review, we present the information on the specifics of COVID-19 pathogenesis and give a theoretical justification for the factors that determine the course of this disease in patients of different age groups, patients with chronic pathology, and pregnant women. Particular attention is paid to the post-COVID syndrome.