1. The Torah forbids a kohen to become tamei through contact with a corpse (tumas-mes). This is one of the 613 commandments.
2. Every Jew, even a non-kohen, is forbidden to cause a kohen to become tamei-mes.
3. This prohibition applies worldwide, even today when there is no Temple, and even when the kohen is already tamei.
4. A kohen who disregards these halachos forfeits the special honor due every kohen. He may not say Birkas Kohanim, and he is not honored by being called up first to the Torah reading until he confirms before a beis din (rabbinical court) that he will avoid tumas-mes henceforth. A kohen who forfeited the special honor due him as a kohen is still bound by the prohibition against becoming tamei.
A Kohen Who Became Tamei
5. A kohen who became tamei from a corpse is prohibited from becoming tamei again.
6. A kohen is forbidden to come in contact with a corpse for even a second, and if he is in a place that has tumas-mes, he must leave immediately.
7. A kohen sleeping in a room where there is tumas-mes must be awakened and warned to leave.
Which Kohanim Are Included in the Prohibition of Those Forbidden to Become Tamei
1. The prohibition of tumas-mes applies to all male kohanim, even those who have physical deformities that disqualify them from Temple service.
2. It is forbidden for any Jew to cause any kohen, even a newborn, to become tamei-mes. One may not even instruct a kohen to make himself tamei.
3. However, one is not obliged to remove a child below the age of chinuch for tumas-mes from a place where there is a corpse, and one does not have to prevent him from entering for his own purposes.
4. The father (and perhaps the mother) of a kohen must train his son to keep away from tumas-mes from the time the child is old enough to be educated in this mitzva. If such a child is sleeping in a room where there is tuma, his father should preferably wake him up and tell him to go outside.
5. A kohen in a place that conveys tumas-mes should not delay leaving in order to remove his son as well. If the tuma is derabbanan, he may be lenient and remove his son.
Those Permitted to Become Tamei
6. The wife and daughter of a kohen may become tamei-mes.
7. A challal (a descendant of a kohen who defiled his status as a kohen in a way that disqualifies him mi’de’Oraisa) is also allowed to become tamei.
8. A kohen’s pregnant wife is allowed to become tamei-mes, but she should consult a halachic authority concerning which hospital is best to give birth in.