According to this meta-analysis, vitamin D is involved in the onset of CAP due to the binding of the active form of vitamin D (1,25OH)2D3, which is deficient in patients with XLH, to its receptor (VDR), which stimulates the expression of antibacterial peptides resisting bacterial and viral infections; moreover, it has been demonstrated that in in vivo experimental models, vitamin D deficiency causes the reduction of the level of VDR, thus damaging the epithelia of the respiratory tract mucous membranes with uncontrolled inflammatory reactions [34]. This evidence concerns the gene VDR and viral infectious disease.