Matthew Poole and continuators on Leviticus 13:13

13 Then the priest shall consider, and behold, if the leprosy have covered all his flesh , he shall pronounce him clean that hath the plague: it is all turned white: he is clean:

(i. e) The sign or appearance of the leprosy: or the scab is called a leprosy, because at first view it seemed to be so to the Priest and to other beholders.

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.