Charles Ellicott Commentary Leviticus 13:51

"and he shall look on the plague on the seventh day: if the plague be spread in the garment, either in the warp, or in the woof, or in the skin, whatever service skin is used for; the plague is a fretting leprosy; it is unclean." — Leviticus 13:51 (ASV)

And he shall look on the plague. —If at the end of the week, when the priest examines it again, he finds that the condition has spread, it undoubtedly indicates malignant leprosy. Here, again, the symptom of spreading is the same in the garment as in the human being. (Leviticus 13:8, and other relevant passages.) The leprous garment, like a human leper, makes everything and everybody unclean by contact with it, or by coming into the house where it remains.