Commentary for Leviticus 13:18

שחין A BOIL — The word expresses the idea of heat. It is so called because the flesh has become inflamed by an injury which came about as the result of a blow and not as the result of fire (the latter case being mentioned in v. 24 and being termed a מכוה) (Chullin 8a).

שחין, our sages explained this as like the infected hot area of an injury on the flesh’s skin. There is another type of שחין, the result of a burning of the skin by fire.

Because of a wound. [This raises a strong difficulty: Why did Rashi not include an inflammation that came by itself and not because of a wound? (Re’m)] It seems Rashi’s intention is that so much more so [there is impurity regarding] an inflammation which occurred by itself and not because of a wound. Rashi only comes to exclude an inflammation that was caused by a fire, since that is called a מכוה (burn). All this is obvious in Toras Kohanim and Chulin (8a). Re’m brought the Gemora in Chulin and the Toras Kohanim but did not realize it is a kal vachomer, and therefore he wrote what he wrote (Nachalas Yaakov).