But if the scab spread much abroad in the skin, after that he hath been seen of the priest for his cleansing, he shall be seen of the priest again.

Jamieson-Fausset-Brown Bible Commentary

7, 8. But if the scab spread much abroad in the skin-Those doubtful cases, when they assumed a malignant character, appeared in one of two forms, apparently according to the particular constitution of the skin or of the habit generally. The one was "somewhat dark" [Le 13:6]-that is, the obscure or dusky leprosy, in which the natural color of the hair (which in Egypt and Palestine is black) is not changed, as is repeatedly said in the sacred code, nor is there any depression in the dusky spot, while the patches, instead of keeping stationary to their first size, are perpetually enlarging their boundary. The patient laboring under this form was pronounced unclean by the Hebrew priest or physician, and hereby sentenced to a separation from his family and friends-a decisive proof of its being contagious.

Matthew Henry's Concise Commentary

13:1-17 The plague of leprosy was an uncleanness, rather than a disease. Christ is said to cleanse lepers, not to cure them. Common as the leprosy was among the Hebrews, during and after their residence in Egypt, we have no reason to believe that it was known among them before. Their distressed state and employment in that land must have rendered them liable to disease. But it was a plague often inflicted immediately by the hand of God. Miriam's leprosy, and Gehazi's, and king Uzziah's, were punishments of particular sins; no marvel there was care taken to distinguish it from a common distemper. The judgment of it was referred to the priests. And it was a figure of the moral pollutions of men's minds by sin, which is the leprosy of the soul, defiling to the conscience, and from which Christ alone can cleanse. The priest could only convict the leper, (by the law is the knowledge of sin,) but Christ can cure the sinner, he can take away sin. It is a work of great importance, but of great difficulty, to judge of our spiritual state. We all have cause to suspect ourselves, being conscious of sores and spots; but whether clean or unclean is the question. As there were certain marks by which to know it was leprosy, so there are marks of such as are in the gall of bitterness. The priest must take time in making his judgment. This teaches all, both ministers and people, not to be hasty in censures, nor to judge anything before the time. If some men's sins go before unto judgment, the sins of others follow after, and so do men's good works. If the person suspected were found to be clean, yet he must wash his clothes, because there had been ground for the suspicion. We have need to be washed in the blood of Christ from our spots, though not leprosy spots; for who can say, I am pure from sin?