CYP27B1 and infectious disease: The important role of vitamin D in host resistance to infection is supported by findings such as 1) immune cells can produce CYP27B1 and convert 25 (OH)D to 1α-25 (OH)2D; 2) Most cells of the immune system express VDR; 3) Production of 1α-25 (OH)2D from the immune system induces the production of anti-bacterial (cathelicidin, beta-defensin); and 4) Impaired vitamin D status contributes to the burden of infectious diseases worldwide [17,18].