Komentarz do Kapłańska 15:24

ותהי נדתה עליו [AND IF ANY MAN LIE WITH HER AT ALL] THEN SHALL HER UNCLEANLINESS OF SEPARATION BE UPON HIM — One might think that he follows in her wake — that if he had intercourse with her e. g. on the fifth day of her separation he shall become unclean for three days only (for the fifth, sixth and seventh) like herself (i. e. because these are the remaining days during which she is unclean)! Scripture, however, states: "he shall be unclean seven days”. Therefore these words cannot refer to the duration of her uncleanness. What then is the force of ותהי נדתה עליו? They mean: her state of uncleanness as a נדה shall be upon him. How is it in the case of her? She, being an אב הטומאה, makes man unclean, and also earthen vessels by means of ‎‎היסט! So, too, he becomes an אב הטומאה (not a ולד הטומאה), and makes man unclean, and also earthen vessels by means‎ of היסט! (Niddah 33a)

אשר ישכב עליו יטמא. This degree of ritual contamination is sufficient to confer further contamination on food and drink coming into contact with it. The severity of contamination of someone lying on top of a menstruating woman is equivalent to that of someone below a male afflicted with the disease of being zav. This is spelled out in Niddah 32.

Perhaps he will join her in her state. Meaning: For her sake, as in the expression (Shemos 11:8): “And all the people that follow you (ברגליך),” which means: that are drawn after you. [Perhaps] he will enter the days of her purity for her sake, i.e., straight away when she enters the days of purity he will immediately follow her. [Therefore,] the verse says...

ישכב איש אותה, “a man will lie with her;” this excludes a woman suffering from the skin disease tzoraat; she does not confer her specific impurity to her partner, as the word אותה, implies a limitation, i.e. “only her, someone in her state.” (Chafetz Chayim)