VDR and infectious disease: The discovery of vitamin D receptors (VDR) in almost all human tissues and the fact that VDR activation regulates gene expression like classic steroid hormones, including hundreds of vitamin D-regulated genes, provides a sound scientific basis to postulate a potential role of vitamin D not only for skeletal diseases but also for many extraskeletal chronic diseases, including cancer, autoimmune, or infectious diseases [1,7,8,9].