פּירוש אויף ויקרא 5:3

בטמאת אדם [OR IF HE TOUCHES] THE UNCLEANNESS OF MAN — This refers to uncleanness resulting from a corpse (i. e. it implies both touching the corpse itself or touching anyone who has come in contact with the corpse) (Sifra, Vayikra Dibbura d'Chovah, Chapter 13 8).

BUT LATER THAT PERSON REALIZES GUILT. It becomes known to him now that he is impure, and he knows that he is guilty.

ונעלם ממנו והוא טמא, “but it was concealed from him,” the same applies when the party asked to swear had forgotten the occurrence which he was now asked to swear about Nachmanides writes that actually, the Torah abbreviated, meaning to tell us that the sin occurred due to the party having forgotten something of relevance. There was no need to spell out the nature of the sin, seeing it was well known (except to the party who had committed it) He would only become guilty of the “sin” of being ritually unclean if he were to enter the precincts of the Holy Temple, or he would attempt to eat sacrificial meat, before either being alerted to it, or undergoing ritual purification in order to be on the safe side. Forgetting an undertaking by an oath is culpable only if in addition to having forgotten about it he had violated the terms of the oath.

לכל טמאתו WHATSOEVER UNCLEANNESS OF HIM IT BE — This is intended to include in this law the uncleanness resulting from touching men or women who have a flux (and those in similar physical condition e. g. ‎'וכו‎ יולדת ‎ נדה‎) Sifra, Vayikra Dibbura d'Chovah, Chapter 13 8.

‎‏אשר יטמא‎ — these apparently redundant words are intended to include in this law one who touches a man who had intercourse with a ה‎‏נד and has not yet immersed himself (cf. Leviticus 15:24) (Sifra, Vayikra Dibbura d'Chovah, Chapter 13 8).