Rambam, Hilchos Aveilus 

Chapter 3: The prohibition for a Kohen to defile himself to the dead

Halacha 1: The prohibition and liability for lashes

• Any Kohen which defiles himself to any corpse other than the six relatives mentioned in the Torah, or his wife, is liable for the penalty of lashes.
• Witnesses and warning: He’s only liable for lashes if he was pre-warned by witnesses who saw him in the act.
• The method of impurity: The above prohibition and liability applies whether one touches a corpse, or enters a tent with a corpse, or carries a corpse.
• The substance of impurity: It likewise applies whether one contacts the actual corpse, or a substance which was expelled from the corpse.

Halacha 2: Touching a grave

• If the Kohen touches a grave he is liable for the penalty of lashes.
• Touching its clothing: It is permitted for a Kohen to touch clothing that were on a corpse, even though he will become impure for seven days.

Halacha 3: Entered an impure Ohel and the status of a Gentile corpse

• A Kohen who enters an impure tent that became impure due to having contained a corpse is liable for lashes.
• This applies even if the impurity has already left the tent, being that the tent itself is still impure.
• The laws relating to the definition of a tent, as well as the substances that are intervene or carry the impurity, have already been explained in the laws of Tumas Meis.
• The corpse and grave of a Gentile: The corpse of a Gentile does not carry impurity to a tent, and hence not only may a Kohen enter a tent which contains the corpse of a Gentile, but he may even come in contact with their graves. He may thus enter a Gentile cemetery and step on their graves. However, he may not touch or carry the actual corpse of a Gentile.

Halacha 4: A Kohen who enters a cemetery or Ohel

• A Kohen who by mistake enters a cemetery or Ohel which contains a corpse, is not held liable if he leaves as soon as he is made aware of his mistake.
• If he delayed his exit: If he was made aware, that if he was prewarned, and did not jump and leave right away and delayed the amount of time it takes to bow down prior to leaving, then he is liable for lashes.
• If he enters and exits many times: A Kohen who enters and exits and then reenters the cemetery, is liable for lashes for each time that he enters the cemetery if he was warned prior to each time.
• If he touches a corpse many times: A Kohen who touched a corpse many times, is liable for lashes for each time that he touched it if he was warned prior to each touching.
• This applies even he did so 100 times, he would be liable for 100 sessions of lashes.
• If he remains in the cemetery or remains with his contact on the corpse: A Kohen who enters a cemetery or Ohel or touches a corpse and continues doing so without stop, is only liable for lashes one time even if he was warned constantly to stop while he continued to remain in the cemetery or contacting the corpse.

Halacha 5: One who causes a Kohen to become impure

• One who causes a Kohen to become impure transgresses a negative command of Lifnei Iver.
• Lashes: If the Kohen had the person intentionally [Bemeizid] defile him to the death, then he is liable for lashes while the perpetrator transgresses the negative command. If, however, the Kohen was not aware of the transgression [Beshogeg], then the perpetrator is liable for lashes

Halacha 6: The prohibition for a high priest to defile himself to any corpse even a relative

• It is forbidden for a high priest to defile himself to any corpse including a relative. Hence, he may not defile himself on behalf of his father or mother.
• Ohel: He may not even enter a tent which contains the corpse of his relative.
• Two transgressions: If the high priest defiles himself to a corpse, it is possible that he transgresses two negative commands, one for defiling himself to the dead and the second being for entering a tent with a corpse, and he would consequently become liable for two sessions of lashes.
• The calculation is as follows: If he simply touches or carries a corpse then he is liable for one session of lashes. If, however, he entered a tent and remaind there after someone died, then he is liable for two sessions of lashes

Halacha 7:

• If prior to the high priest entering the tent which contains a corpse, he defiled himself to a corpse, then he is liable for two sessions of lashes.

Halacha 8: A Meis Mitzvah

• It is a mitzvah and command for one to help to bury a Meis Mitzvah.
• This applies even for a Kohen and even for the high priest. The oral tradition maintains that even they are commanded to defile themselves to a corpse who is a Meis Mitzvah.
• What is a Meis Mitzvah? A Meis Mitzvah is a Jewish corpse that was found thrown on the road which does not have anyone to bury him other than the person who met the corpse.
• If the priest is with other people: If the priest met the corpse together with other people, then he may not defile himself to the corpse and is to rather let them deal with it.
• If the priest can call other people: If the priest is able to call other people to bury the corpse, then he may not defile himself to the corpse and is to rather call other people and let them deal with it.

Halacha 9: Precedents in dealing with a Meis Mitzvah

• A Kohen versus a Nazir by a Meis Mitzvah: If a Kohen and Nazir were walking together when they bumped into a Meis Mitzvah, then the Nazir is to deal with the corpse and not the Kohen, being that his prohibition is only temporary.
• A Kohen Hedyot versus a Kohen Gadol by a Meis Mitzvah: If a Kohen Hedyot and Kohen Gadol were walking together when they bumped into a Meis Mitzvah, then the Kohen Hedyot is to deal with the corpse and not the Kohen Gadol.
• The general rule is that whoever has a lower status of holiness is to be the one to deal with the corpse.
• A Segan Kohen Gadol versus a Meshuach Milchama by a Meis Mitzvah: If a Segan Kohen Gadol and Kohen Meshuach Milchama were walking together when they bumped into a Meis Mitzvah, then the Meshuach Milchama is to deal with the corpse and not the Segan Kohen Gadol.

Halacha 10: A Kohen defiling himself to a Nassi

• If a nasi passes away, then all the Jewish people, including even a Kohen, are to defile themselves on his behalf, as is the rule by a Meis Mitzvah.

Halacha 11: The law of a Kohenes and Chalal

• Kohenes: The daughters of Kohanim are not under a prohibition against defiling themselves to the dead.
• Challal: It is permitted for a Kohen who is a Challal to the final himself to the dead.

Halacha 12: The law of a Kohen who is a child

• Causing him to become impure: It is forbidden for an adult to defile a child who is a Kohen.
• Stopping him from becoming impure: There is no obligation upon the courts to stop a child who is a Kohen from defiling himself to a corpse, however his father must educate him against doing so.

Halacha 13: Rabbinical impurities

• The four cubits of the corpse: It is rabbinically forbidden for a Kohen to enter within the four cubits parameter of a corpse, and if he does so he is liable for rabbinical lashes.
• Beis Haperas: It is rabbinically forbidden for a Kohen to enter a cemetery which is defined as a Beis Haperas, and if he does so he is liable for rabbinical lashes.
• The Diaspora: It is rabbinically forbidden for a Kohen to leave the land of Israel to the Diaspora, and if he does so he is liable for rabbinical lashes.
• Mixtures of bloods: It is rabbinically forbidden for a Kohen to defile himself to a mixture of bloods [i.e. Da Tevusa], and if he does so he is liable for rabbinical lashes.
• Gravestone: It is rabbinically forbidden for a Kohen to touch a lone gravestone, and if he does so he is liable for rabbinical lashes. If, however, he enters a cemetery then he is biblically liable for lashes.
• All the above are considered primary rabbinical impurities.

Halacha 14: Cases of exception by a rabbinical impurity

• Entering Diaspora for sake of mitzvah: It is permitted for a Kohen to enter a Beis Haperas or leave to the Diaspora for the sake of a mitzvah, if there is no other way to fulfill the mitzvah.
• For example, he may go to the Diaspora for the sake of marrying a woman or learning Torah.
• This applies even if he is able to learn Torah in Israel, as one does not merit to learn from every individual.
• Defiling oneself to a rabbinical impurity: It is permitted for a Kohen to defile himself to a rabbinical impurity for the sake of respect of humanity.
• For example, he may enter the Beis Haperas for the sake of escorting a mourner.
• Likewise, he may skip on top of graves for the sake of greeting a king, whether of Israel or another nation.
• Likewise, he may defile himself with a rabbinical impurity for the sake of participating in a lawsuit against Gentiles.