CYP27B1 and viral infectious disease: Recent studies on the regulation of immune function with SARS-CoV-2 infection demonstrated how the expression of the enzyme CYP27B1 by the airway epithelium and alveolar macrophages that produce the active metabolite of vitamin D (1,25 (OH)2D) and the vitamin D receptor (VCR) that promotes the innate immune response [43]) influences the severity of the viral infection.