Mendelian susceptibility to mycobacterial diseases (MSMD) is a genetic disorder caused by partial STAT1 deficiency. It is not contagious as it is an inherited condition resulting from specific gene mutations. MSMD affects the immune system's ability to fight mycobacterial infections, such as tuberculosis. Individuals with MSMD are more susceptible to these infections, but it does not spread from person to person like a contagious disease.