(a) The SHA'AGAS ARYEH answers that the Isur of Nezirus to become Tamei will
take effect upon the Isur of a Kohen to become Tamei through the mechanism
of "Isur Kolel." When the Isur of Nezirus takes effect, it includes more
than just the Isur of Tum'ah; it includes an Isur to shave and an Isur to
eat grape products. Hence, since it takes effect with regard to those other
items, it also takes effect with regard to Tum'ah.
However, the AVNEI MILU'IM (Teshuvah 15) asks that, normally, two different
types of Isur cannot constitute an "Isur Kolel." That is, an Isur must
prohibit two of the *same type* of item in order to be considered an "Isur
Kolel," such as an Isur which prohibits two items of *food*, or two forms of
*Tum'ah.* An Isur which prohibits food and also prohibits Tum'ah (like the
Isur of Nezirus) cannot be considered an "Isur Kolel."
The Avnei Milu'im therefore suggests that the "Isur Kolel" of a Kohen Hedyot
who becomes a Nazir is that the Isur of Nezirus makes him prohibited to be
Metamei even for close relatives (for whom he was permitted to be Metamei as
a Kohen Hedyot). Since the Isur of Nezirus prohibits him from becoming Tamei
from those relatives, we would think that it also prohibits him, through
"Isur Kolel" to become Tamei to a Mes Mitzvah.
However, this does not explain why the Isur Tum'ah of Nezirus should take
effect on a Kohen Gadol, because a Kohen Gadol is prohibited to be Metamei
for close relatives even without the Isur of Nezirus! The HAGAHAH on the
Avnei Milu'im writes that perhaps the Gemara's case of a Kohen Gadol who
becomes a Nazir is referring to a Kohen who is not yet a Kohen Gadol who
accepts upon himself to become a Nazir at the exact moment that he is
appointed as Kohen Gadol. Hence, the Isur Tum'ah of Nezirus is able to take
effect simultaneously with the Isur Tum'ah of a Kohen Gadol, since they take
effect "b'Bas Achas" and it is not a case of "Isur Chal Al Isur."
The KEREN ORAH suggests another reason to explain why the Isur Tum'ah of
Nezirus takes effect on the Isur Tum'ah of Kehunah. He refers the Gemara
earlier (4a) which cites a verse that teaches that Nezirus is an exception
to the rule of "Ein Isur Chal Al Isur." For this reason, the Gemara there
says that the Isur of a Nazir to drink wine can take effect on a
pre-existing Isur on the person to drink wine (such as when the person had
made a Shevu'ah not to drink wine, according to Rebbi Shimon according to
Tosfos there; see Insights there). Similarly, here, too, the Isur Tum'ah of
a Nazir can take effect on the pre-existing Isur of a Kohen to become Tamei.
(b) The Mitzvah of a Kohen Gadol to shave each week is only a Mitzvah
d'Rabanan. It is not a Mitzvah d'Oraisa nor a Halachah l'Moshe mi'Sinai.
Therefore, the Isur d'Oraisa of a Nazir shaving overrides the Mitzvah
d'Rabanan of a Kohen Gadol to shave each week.
Alternatively, according to the KEREN ORAH cited above, it is possible to
learn the laws of the Isur of Tum'as Mes of a Nazir from the laws of the
Isur of wine. Just like the Isur of Nezirus overrides a Mitzvah to drink
wine (such as when the person had made a Shevu'ah to drink wine), so, too,
the Isur of Nezirus overrides the Mitzvah of a Kohen Gadol to shave his hair
once a week.
However, this answer will not work here, because when it comes to the Isur
of Gilu'ach, there is a verse which teaches that a Gilu'ach of a Mitzvah
(such as the Mitzvah of a Metzora to shave) *does* override the Isur of a
Nazir to shave, in the case of a Nazir who becomes a Metzora. This is
derived from the word "Rosho" in the verse discussing the Metzora's Mitzvah
to shave (Vayikra 14:9). The Gemara earlier (44a) discusses the difference
between a Mitzvah to do Gilu'ach, which does override the Isur of a Nazir to
shave, and a Mitzvah to drink wine, which does *not* override the Isur of a
Nazir to drink wine.
Therefore, if the Mitzvah for a Kohen Gadol to shave his hair each week is
mid'Oraisa, then indeed a Nazir who is a Kohen Gadol will have to shave his
hair each week (since the Mitzvah of the Kohen Gadol to shave overrides the
Isur of a Nazir to shave), and at the end of his Nezirus he will have to
step down from the position of Kohen Gadol in order to grow his hair for
thirty days so that he can perform the Gilu'ach of a Nazir and bring his