What is the difference between a "Sh'chin" and a "Michvas Eish" (in Pasuk 24)?
Rashi and Rashbam (citing Nega'im, 9:1): Both are the result of heat, but whereas the former is caused by a stroke (by a heavy object), 1 the latter is caused by fire.
Incorporating any wound that is not caused by fire, even one that comes naturally (See Sifsei Chachamim).
Why does it say "u'Vasar
Da'as Zekenim: Toras Kohanim expounds, if there was a swelling due to a blow, it can receive a Nega only if afterwards the flesh healed (the swelling went down) and a scab formed.
Kli Yakar (from Sotah 5a): Refu'ah was not said about the person, rather, the flesh. [Only] one who has a [soft] heart of flesh, there is a cure for him.