Commentary for Leviticus 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).

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.

One who swallows the carcass. This causes a person to become impure in the place of swallowing [i.e., when the carcass that is swallowed touches the esophagus] to make garments [that he is wearing] impure; if he eats it and enters the Temple he is liable. You might ask: The word בה [“אשר יטמא בה (that causes impurity within)”] is an exclusion! They already answered in the Gemara (Shavuos 7b) that it is because there is an extra exclusion — for it is written [here] in the section of the sliding scale offering (i.e., a sacrifice whose worth varies in accordance with the financial ability of the person bringing it): “Or, if he touches the impurities of man.” Thus, something [which imparts impurity] through touching is included, but what is not [capable of making impure] through touching is not. The carcass of a kosher bird does not impart impurity through touching, if so, why does the Merciful One [in the Torah] write “within”? Hence, it is an “exclusion after an exclusion,” and “an exclusion after an exclusion” only comes to be inclusive.