Mendelian susceptibility to mycobacterial diseases (MSMD) is a rare immunodeficiency syndrome, characterized by a narrow vulnerability to poorly virulent mycobacteria, such as bacillus Calmette-Guérin (BCG) vaccines and environmental mycobacteria (EM), and defined by severe, recurrent infections, either disseminated or localized. The disease is Mendelian susceptibility to mycobacterial diseases (Orphanet_748, an Orphanet rare-disease nomenclature id). Also known as: Idiopathic infection caused by BCG or atypical mycobacteria, MSMD, Mendelian susceptibility to atypical mycobacteria, Mendelian susceptibility to mycobacterial infections.