レビ記 15:2

כי יהיה זב WHEN THERE BE A RUNNING ISSUE — One might think if there be a running issue from any portion of his body (e. g., ear, nose) he shall be unclean! Therefore it states מבשרו “from his flesh" (i.e. from some part of his flesh) and not from all parts of his flesh (the prefix מ is a partitive מ). Now, however, that Scripture has made a difference between one part of his flesh and another part of his flesh, I have the right to draw a conclusion: it (the Torah) pronounces uncleanness in a case of a man with a running issue (זב) and it pronounces uncleanness in a particular case of a woman with a running issue (זבה). How is it in the case of a זבה? Through that place in her body where she becomes unclean with a lighter uncleanness, viz., that of a נדה, she also becomes unclean with a more stringent uncleanness i.e. through זיבה, a continuance of the issue beyond the usual period! Similarly the זב: through the place where he becomes unclean with a lighter uncleanness, viz., קרי, (an involuntary emission of semen which makes him unclean only until evening; cf. v. 16), he also becomes unclean through a more stringent uncleanness, viz., זיבה (when he is unclean seven days and must bring a sacrifice; cf. vv. 13 ff. with 16 ff.; from the latter it is evident which part of the body is in question; it follows logically that the same part is here intended) (Sifra, Metzora Parashat Zavim, Section 1 5). טמא‎ זובו HIS ISSUE IS UNCLEAN — This teaches regarding a drop (i.e. the semen itself) that it communicates uncleanness (and it is not only the man that has the issue who is unclean) (Sifra, Metzora Parashat Zavim, Section 1 8-9; Niddah 55a). The זב issue referred to here is similar to water of dough (that runs out of dough) made of barley and it is dissolved (fluid) being similar to the white of an addled egg (where the white and the yolk have run together); שכבת זרע, however, is viscid and looks like the white of an egg that has not become addled (Niddah 35b).

דַּבְּרוּ֙ אֶל־בְּנֵ֣י יִשְׂרָאֵ֔ל וַאֲמַרְתֶּ֖ם אֲלֵהֶ֑ם אִ֣ישׁ אִ֗ישׁ כִּ֤י יִהְיֶה֙ זָ֣ב מִבְּשָׂר֔וֹ זוֹב֖וֹ טָמֵ֥א הֽוּא׃

Speak to the children of Yisra᾽el, and say to them, When any man has a running issue out of his flesh, because of his issue he is unclean.