Positive commandment · Mitzvat Aseh #101 of 613
For a person afflicted with tzara’at to be impure and to impart impurity
להיות המצורע טמא ומטמא
The verse · Vayikra 13:2
אָדָם כִּי־יִהְיֶה בְעוֹר־בְּשָׂרוֹ שְׂאֵת אוֹ־סַפַּחַת אוֹ בַהֶרֶת וְהָיָה בְעוֹר־בְּשָׂרוֹ לְנֶגַע צָרָעַת וְהוּבָא אֶל־אַהֲרֹן הַכֹּהֵן אוֹ אֶל־אַחַד מִבָּנָיו הַכֹּהֲנִים
“When a man shall have in the skin of his flesh a swelling, a scab, or bright spot, and it be in the skin of his flesh the plague of żara῾at; then he shall be brought to Aharon the priest, or to one of his sons the priests:”
The Rambam’s words
For a person afflicted with tzara’at to be impure and to impart impurity.
להיות המצורע טמא ומטמא.
Why · Sefer HaChinuch #169
Matter of the impurity of a metzora
“It is from the roots of the commandment to fix in our hearts that the providence of G-d, blessed be He, is individualized upon everyone among people, and that His eyes are observing all of their ways, as it is written (Job 34:21), “For His eyes are upon a man’s ways; all of his steps He sees.””
Who & when
“And the laws of tzara’at are practiced by males and females at all times when there are expert priests seeing them.”
Where in the Mishneh Torah
Hilchot Defilement by Leprosy הלכות טומאת צרעת