The supposed impurity or impurities of various metals, understood as causing their deviation from gold.
An infectious disease caused by the bacterium Mycobacterium leprae, gradually producing nerve damage and patches of skin necrosis and historically handled by permanently quarantining its sufferers.
Similar contagious skin diseases causing light patches of scaly skin, particularly psoriasis, syphilis, vitiligo, scabies, and (biblical) the various diseases considered "tzaraath" in the Old Testament of the Bible.
Anything considered similarly permanent, harmful, and communicable, particularly when such a thing should be handled by avoidance or isolation of its victims.