If a Nazir walks underneath the same roof as a Mes he is punished only once with Malkus. (1)
If a Nazir enters under the roof of a Mes in a chest and the top of the chest is removed, the Nazir is punished with two Malkus. (2)
If a Nazir enters under the roof of a Goses (deathly ill person) and the person subsequently dies the Nazir is punished with two Malkus.
Reish Lakish says that it is a Machlokes Tana'im whether a Kohen may come into contact with a Goses.
A Kohen may become Tamei to a Metzora and a Zav.
If a Kohen had a Mes on his shoulder and he was handed another Mes he is only punished once with Malkus.
A person is not Metamei until he dies even if he is chopped up or is a Goses. (3)
The Chachamim hold that a Kohen may not become Tamei to the severed head of his father.
A Kohen may become Tamei to a Mes Mitzvah.
If a person dies in a place where there is no one around to bury him it is regarded as a Mes Mitzvah.
If the Mes is in a place that if he was able to call out someone would answer he is not regarded as a Mes Mitzvah..
If a person is killed on the road it is regarded as a Mes Mitzvah.
R. Yehudah says that if the Kohen is already Tamei to his relative he may also become Tamei to a severed limb of that relative.
A Kohen may not become Tamei to a k'Zayis Mes of one of his relatives. (4)
R. Eliezer Ben Yakov says that a Kohen may become Tamei to the spine, skull, the Rov Binyan (two thighs and a hip) or Rov Minyan (125 limbs) of his relative.
A BIT MORE
1. He is only punished for the prohibition of Lo Yetamei not for the prohibition of Lo Yavo because as soon as his finger is under the roof he is Tamei and when he subsequently walks in he is already Tamei.
2. As long as he is in the chest he is not susceptible to Tum'ah and as soon as the top of the chest is removed he immediately transgresses both the prohibition of Lo Yetamei and the prohibition of Lo Yavo.
3. However according to Reish Lakish it is forbidden for a Kohen to come into contact with a Goses according to the opinion of one Tana.
4. He also may not become Tamei to a k'Zayis Netzel (liquid that comes from a Mes), or a spoonful of decomposition.
The Gemara says that if a Nazir enters underneath the roof of a Mes as soon as one finger enters under the roof he is Tamei from the Mes and he no longer will liable for the prohibition of Lo Yavo. Tosfos asks why does the Gemara not state a case that the Nazir entered backwards underneath the roof of the Mes. Tosfos answers that entering backwards is not regarded as entering. The Orach Mishor explains that it is not regarded as entering only with respect to the prohibition of Lo Yavo. However, with respect to the prohibition of Lo Yetamei he becomes Tamei when he enters because we look at the house as if it is completely filled with Tum'ah.
THE TUM'AH OF A KOHEN
A Kohen may not enter a Beis ha'Kevaros even for one of his own relatives, because if after he becomes Tamei to his relative he subsequently comes into contact with another Mes he is punished with Malkus. (Rosh)