Positive commandment · Mitzvat Aseh #102 of 613
For a garment afflicted with tzara’at to be impure and to impart impurity
להיות בגד מנוגע טמא ומטמא
The verse · Vayikra 13:47
וְהַבֶּגֶד כִּי־יִהְיֶה בוֹ נֶגַע צָרָעַת בְּבֶגֶד צֶמֶר אוֹ בְּבֶגֶד פִּשְׁתִּים
“The garment also in which is the plague of żara῾at, whether it be a woollen garment, or a linen garment;”
The Rambam’s words
For a garment afflicted with tzara’at to be impure and to impart impurity.
להיות בגד מנוגע טמא ומטמא.
Why · Sefer HaChinuch #172
Matter of ailments of clothing
“It is from the roots of the commandment that G-d wanted in His great goodness to chasten us, ‘like a man chasten his child.’ As this matter is not natural, but [rather] a sign with the holy people, in order that they learn and take rebuke in the changing of the items that are particularly for their use — and they are clothes of wool and flax, as most usage of people is with them — and they repent through this from their evil way, before the tzara’at breaks out also in their bodies.”
Who & when
“And [it] is practiced with the clothes of males and females.”
Where in the Mishneh Torah
Hilchot Defilement by Leprosy הלכות טומאת צרעת