moral infections

That it is at least as difficult to stay a moral infection as a physical one; that such a disease will spread with the malignity and rapidity of the Plague; that the contagion, when it has once made head, will spare no pursuit or condition, but will lay hold on people in the soundest health, and become developed in the most unlikely constitutions: is a fact as firmly established by experience as that we human creatures breathe an atmosphere. A blessing beyond appreciation would be conferred upon mankind, if the tainted, in whose weakness or wickedness these virulent disorders are bred, could be instantly seized and placed in close confinement (not to say summarily smothered) before the poison is communicable.

— Dickens, Little Dorrit