The 75th prohibition is that a kohen who is tameh is forbidden from participating in the Temple service while in a state of tumah.
The source of this prohibition is G‑d's statement to the kohanim,1 "Be careful regarding the sacred offerings of the Israelites, and do not desecrate My holy Name."
In the ninth chapter of Sanhedrin2 our Sages explained, "What is the source for the law that one who serves in the Temple while tameh is punishable by death? It is written, 'Tell Aaron and his sons, "Be careful regarding the sacred offerings of the Israelites, and do not desecrate (v'lo y'chal'lu)," and elsewhere it is written,3 'Because they have desecrated (y'chal'luhu) they shall die for it.'" I.e., just as the punishment for that desecration is misah biy'dei shamayim, so too in this prohibition, "do not desecrate My holy Name" — if one did desecrate by performing the service while in a state of tumah he is punishable by misah biy'dei shamayim.
