VDR and rickets: Furthermore, the presence of VDR is already well documented in the cells of various organs, and it was established that proper function of musculoskeletal, immune, nervous, and cardiovascular systems as well as regeneration of epithelial barriers strongly depends on vitamin D. Vitamin D deficiency (25(OH)D3 level below 20 ng/mL) was not only described as a risk factor for rickets or osteoporosis [39,42] but also impairs the function of the immune system [43,44,45,46,47] and its response to pathogens, including influenza viruses [48] and coronaviruses (SARS-CoV-2) [49].