Adam Clarke on Leviticus 14:34

When ye be come into the land - and I put the plague of leprosy - It was probably from this text that the leprosy has been generally considered to be a disease inflicted immediately by God himself; but it is well known that in Scripture God is frequently represented as doing what, in the course of his providence, he only permits or suffers to be done. It is supposed that the infection of the house, as well as of the person and the garments, proceeded from animalcula. See Clarke's note on Lev 13:47, and Lev 13:52 (note).