Rashi on Leviticus 21:1/רש"י על ויקרא כ״א:א׳

1אמר אל הכהנים SAY UNTO THE PRIESTS [… AND THOU SHALT SAY UNTO THEM] — “Say" and again “thou shalt say unto them" — this repetition is intended to admonish the adults about their children also — that they should teach them to avoid defilement (Yevamot 114a).

2בני אהרן THE SONS OF AARON — One might think that חללים (priests who have lost their priestly status for reasons connected with their birth or marriage) also may not defile themselves by the dead, Scripture therefore states, "Say unto the priests", — thus only those sons of Aaron are included who have not lost their priestly character; consequently חללים are excluded (Sifra, Emor, Section 1 1) .

3בני אהרן THE SONS OF AARON — This implies also those of Aaron's sons who have a bodily blemish;

4בני אהרן THE SONS OF AARON — but not the daughters of Aaron (Sifra, Emor, Section 1 1).

5לא יטמא בעמיו THERE SHALL NONE BE DEFILED BY THE DEAD AMONG HIS PEOPLES — This means, as long as the dead is among his peoples (i. e. so long as there are some of his people — Jews — who can occupy themselves with his burial) thus excluding the case of a מת מצוה (a corpse of a person whose relatives are unknown or which lies in a place where there are no Jews, nor are there any in the near vicinity; cf. Nazir 43b) in which case the priest is allowed to make himself unclean by handling the corpse (Sifra, Emor, Section 1 3).