John Gill Bible Commentary

And if the priest see that, behold, the scab spreadeth in the skin,.... Is not at a stay, as when he looked at it a second and third time: then the priest shall pronounce him unclean; a leprous person; to be absolutely so, as Jarchi expresses it; and so obliged to the birds (to bring birds for his cleansing), and to shaving, and to the offering spoken of in this section, as the same writer observes: it is a leprosy: it is a clear and plain case that it was one, and no doubt is to be made of it, it is a spreading leprosy: as sin is; it spreads itself over all the powers and faculties of the soul, and over all the members of the body; and it spreads more and more in every stage of life, unless and until grace puts a stop to it.