Jamieson, Fausset and Brown on Leviticus 13:7-8

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.