He is to burn the fabric, weave, or knit, whether the contaminated item is wool or linen or leather. Since the mildew is harmful, the article must be burned up.
This is the law concerning the plague of leprosy in a garment, whether linen or woollen. A leprosy in a garment, with discernible indications of it, the colour changed by it, the garment fretted, the nap worn off, and this in some one particular part of the garment, and increasing when it was shut up, and not to be got out by washing is a...
Commenting on Leviticus 13:47-59
Then the priest shall command that they wash the thing wherein the plague is,.... The priest did not wash it himself, but ordered others to do it; and this was either the part in which the plague was, or the whole garment or skin in which it was; which may be typical of the washing of the garments of men in the blood of Christ...
He shall therefore burn that garment - There being scarcely any means of radically curing the infection. It is well known that the garments infected by the psora, or itch animal, have been known to communicate the disease even six or seven years after the first infection. This has been also experienced by the sorters of rags at some paper mills.