Komentarz do Kapłańska 22:5

בכל שרץ אשר יטמא לו [OR WHOSOEVER TOUCHETH] ANY PROLIFIC CREATURE BECAUSE OF WHICH (לו) HE MAY BE MADE UNCLEAN — This means: who touches any שרץ of the minimum size that is capable of communicating uncleanness, viz., a particle of the size of a lentil (cf. Sifra, Emor, Chapter 4 4; Chagigah 11a).

A lentil. Otherwise Scripture should have written only “Anyone who touches any creeping creature,” and nothing more. Why does it write “that causes impurity to him”? So that you do not say that [he is impure] only if he touched a complete creeping creature.

אשר יטמא לו, “whereby he may become ritually unclean;” the word לו in this verse means בשבילו, “on its account,” on account of the creeping animal. It is parallel to Exodus 1,10, where Pharaoh exerted his people to outsmart the Israelites since he could not outnumber them and said: הבה נתחכמה לו, “let us instead try to be smarter than it,” i.e. “on account of its numerical superiority.” It is also similar to Esther 6,4: אשר הכין לו, “which he had prepared on his account.” (On Mordechai’s account, in order to hang him from that gallows). Our author cites more parallels.