TT #126 · Parshas Tazria · Aliyah 2 · Passuk 10
English: If the eruption spreads out in the skin, so that the eruption covers all the skin of the affected person from head to foot, as far as the priest can see,
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:12
Torah Temimah cites Sifra (Toras Kohanim) for excluding active boils and burns from skin that can receive a tzaraas lesion.
Sifra — Vayikra 13, Midrash Sefer
English: “The whole skin of the lesion” means skin capable of receiving a lesion; this excludes an active boil and an active burn.
Textual derivation
The verse concerns skin capable of receiving a tzaraas lesion, not skin currently occupied by an active boil or burn.