Charles Ellicott Commentary Leviticus 13:7-8

"But if the scab spread abroad in the skin, after that he hath showed himself to the priest for his cleansing, he shall show himself to the priest again: and the priest shall look; and, behold, if the scab be spread in the skin, then the priest shall pronounce him unclean: it is leprosy." — Leviticus 13:7-8 (ASV)

But if the scab spread. —As Leviticus 13:5 prescribes that the priest who examines the patient after seven days’ quarantine, and finds no spreading of the affected spot, is to give another seven days’ quarantine, these verses declare what the examining priest is to do when he notices that the spot has spread.

For his cleansing. —That is, for the purpose of being declared clean. If, after he had appeared before the priest to be examined and declared not leprous, at the end of the first week of seclusion the priest finds that the spot has spread, he must pronounce him unclean, since the spreading indicates that it is leprosy.