Why does the Torah insert the (otherwise superfluos) word "Bo be'Oro Sh'chin"?
Sifra: To incorporate in the Din of Sh'chin where the Nega has nowhere to spread - because it already covers the entire body - even if both the Sh'chin and the Baheres cover the entire body, or if the Sh'chin does not but the Baheres does, or vice-versa. 1
What is the difference between a "Sh'chin" and a "Michvas Eish" (in Pasuk 24)?
Rashi and Rashbam: Both are the result of heat, but whereas the former is caused by being struck (by a heavy object), 1 the latter is caused by fire.
Incorporating any wound that is not caused by fire, even one that comes not vi a stroke (See Sifsei Chachamim).
Why does the Torah insert the word "Sh'chin" both here and in Pasuk 19?
Sifra: To teach us that the Din of Sh'chin applies whether the 'boil' grew by itself or whether the Metzora hurt himself on a piece of wood, on the hot peat of olive-peels, in the Chamei Teveryah or in any other manneer other than fire.
What are the connotaions of Sh'chin ve'Nirpa"?
Sifra: It means that the boil is partially healed and is no longer full of pus, but not fully healed (in that a crust has grown over the location of the boil), in which case it would no longer be called a 'Sh'chin'.
What is the significance of the fact that the boil healed?
Da'as Zekenim: Because, as the Sifra explains, if there was a swelling due to a blow, it is subject to a Nega only if afterwards the flesh healed (the swelling went down) and a scab formed.
Kli Yakar (citing Sotah 5a): Refu'ah was not said regarding the person, but regarding the flesh. This hints that oly someone who has a [soft] heart of flesh, is subject to a 'cure' for his sins.
