Vitamin D has a large number of biological effects due to the effect on the vitamin D receptor, which is present in most tissues in the body. The possible role of vitamin D in infections is explained by its effect on the mechanisms of the innate and acquired immune response. Suppression of the inflammatory response is also an important effect of vitamin D. Many scientists strongly believe that vitamin D deficiency is among the so-called «seasonal stimulators» of acute respiratory viral infections (ARVI), the potential for the prophylactic and therapeutic use of vitamin D in the season of ARVI and influenza is of particular interest. 25-hydroxycalciferol-stimulated production of antimicrobial peptides, such as defensin and cathelicidin is the most important fact proving the possible protective role of vitamin D in influenza and other acute respiratory infections. These endogenous peptides have a direct action, destroying not only microbial pathogens, but also viruses, including the influenza virus.