Commentaire sur Le Lévitique 22:6

נפש אשר תגע בו THE SOUL WHICH HATH TOUCHED IT — i. e. who touches any one of these unclean persons or things (enumerated in the second half of v. 4 and in v. 5).

Any of these [ritually] unclean [persons]. I.e., the verse should have said, “Anyone touching them.”

כי אם רחץ בשרו במים, “unless he has first washed his body in water. (ritual bath) In case we understood this as piecemeal washing of the limbs of one’s body, the Torah adds the word: במים in water (all at the same time). This is why the words: “when the sun sets he will be ritually clean,” is of importance here. Just as sunset is not a gradual process but it occurs at the same time, so the whole body has to be washed at the same time. (Sifra)