In the experiment, 160 medicinal leeches of the species Hirudo verbana Carena, 1820 were studied. Medicinal leeches were fed on the blood of animals and people (conditionally healthy and diseased). Four leeches were taken from each animal/person. The animals were studied for 3 weeks. Mortality was mostly observed in the first days after feeding on the blood of the host. We noted mortality, the appearance of constrictions on the leeches' body, the intensity of the host blood spitting from their body. The host's blood was taken from their stomach on the first day after feeding. Hematological and immunological indicators of blood were determined in the taken blood of the host. As a result of the study of the blood of the sick, significant changes were found, compared to conditionally healthy ones. It was manifested by an increase in erythrocytes and leukocytes. The leukocyte formula looked like in most pathological conditions of the inflammatory process. The obtained indicators of the experiment make it possible to quickly assess the presence of physiological disorders in the early stages of the disease.