This shall be the law of the metzora on the day he is purified… (Leviticus 14:2)
Tzara’as was purified using a particular set of prescribed ingredients: cedar, hyssop, scarlet thread, spring water and, in the case of people and houses but not garments, two birds. Water was commonly used in purification processes, such as through immersion in a mikvah; the other ingredients are unique to the case of tzara’as. (We’ll discuss their significance below.)
The purification process is as follows: after the metzora is pronounced free of tzara’as, the kohein fills a clay bowl with running spring water. He brings two uncaged, unblemished birds, one of which is slaughtered over the bowl of water, then buried. He wraps the cedar and hyssop with the scarlet thread and places them alongside the live bird, all of which are then dipped in the water. The kohein sprinkles the metzora seven times on the back of his hand and the bird is released. (If it returns, the kohein has to send it away – even repeatedly, until it stays away.) Then, the metzora is shaved all over. He immerses his clothes and himself in a mikvah, after which he can re-enter the city, but he is not allowed to have marital relations for a week. During that week, he can still transmit impurity through contact.
At the end of the week, the metzora returns to the kohein. He is shaved again (as we’ll see in the next mitzvah), he immerses his clothes and immerses himself. After that he no longer transmits impurity to others. After the sun sets, he’s completely purified and may eat from sacrifices.
The ingredients have symbolic meanings. For example, the Midrash suggests that the metzora was formerly arrogant like a cedar and he had to learn to humble himself like a hyssop. Similarly, the crimson thread is called tola’as in Hebrew, related to the word for a lowly worm. The Talmud in Arachin (16b) suggests that the birds are brought because it was his squawking gossip that caused him to become a metzora in the first place.
This mitzvah applied to both men and women when there were kohanim who could diagnose tzara’as, which, as we have said, is no longer the case. (Certain aspects did not apply to women. For example, they were not barred from intimate relations for the week.) The details of this mitzvah are discussed in the Mishna in the fourteenth chapter of tractate Negaim; in the Talmud, see tractate Sotah, pages 15a-16b. This mitzvah is codified in the Mishneh Torah in the eleventh chapter of Hilchos Tumas Tzara’as and is #110 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.