Rashi on Leviticus 13:18/רש"י על ויקרא י״ג:י״ח

1שחין 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).

2ונרפא AND IT BECAME HEALED — the boil (not the flesh also mentioned in this verse) became healed and in its place it produced another plague (the שאת mentioned in the next verse).