Leviticus 13:13

If the leprosy have covered all his flesh, he shall pronounce him clean - Why is it that the partial leper was pronounced unclean, and the person totally covered with the disease clean? This was probably owing to a different species or stage of the disease; the partial disease was contagious, the total not contagious. That there are two different species or degrees of the same disease described here, is sufficiently evident. In one, the body was all covered with a white enamelled scurf; in the other, there was a quick raw flesh in the risings. On this account the one might be deemed unclean, i. e., contagious, the other not; for contact with the quick raw flesh would be more likely to communicate the disease than the touch of the hard dry scurf. The ichor proceeding from the former, when brought into contact with the flesh of another, would soon be taken into the constitution by means of the absorbent vessels; but where the whole surface was perfectly dry, the absorbent vessels of another person coming in contact with the diseased man could imbibe nothing, and therefore there was comparatively no danger of infection. Hence that species or stage of the disease that exhibited the quick raw rising was capable of conveying the infection for the reasons already assigned, when the other was not. Dr. Mead thus accounts for the circumstance mentioned in the text. See on Leviticus 13:18 (note). As the leprosy infected bodies, clothes, and even the walls of houses, is it not rational to suppose that it was occasioned by a species of animalcula or vermin burrowing under the skin? Of this opinion there are some learned supporters.

13:13 All his flesh - When it appeared in some one part it discovered the ill humour which lurked within, and withal the inability of nature to expel it; but when it overspread all, it manifested the strength of nature conquering the distemper, and purging out the ill humours into the outward parts.