Parashas Metzora · Passuk 15:33
Chabad.org — English translation + Rashi
Torah Temima points to Niddah 54b (Chizkiyah).
English: “And the one who is ill in her niddah” — it was taught: From where [do we know] that niddah blood conveys impurity? Chizkiyah said: Scripture says “and the one who is ill in her niddah” — the illness (blood) is like her: just as she conveys impurity, so does the blood.
Blood itself impure
Chazal derive that niddah blood is itself a source of impurity, not only the woman.