Midrasz do Kapłańska 15:2

1) (Vayikra 15:2) ("Speak to the children of Israel, etc.") The children of Israel become tamei with zivah (a type of genital discharge); gentiles do not become tamei with zivah (by edict of the Torah). But even though they do not become tamei with zivah, they confer tumah as zavim (by Rabbinical enactment). And terumah is burned because of them (by their touching it), and there is no liability because of them for entering the sanctuary (i.e., If a gentile zav touched a Jew and he entered the sanctuary unwittingly, he is not liable to bring an offering.) If "the children of Israel," this tells me only of (native) Israelites. Whence do I derive for inclusion proselytes and bondsmen? From "and you shall say to them." "a man": This tells me only of a man. Whence do I derive a minor for inclusion? From "a man, a man." These are the words of R. Yehudah. R. Shimon b. R. Yochanan b. B'rokah says: It is written (Vayikra 15:22) "and of one who flows his flux for male or for female." "for male": anyone (even a minor) who is a male. "or for female": whether grown or a minor.

2) "if there be a flux (zov) from his flesh": from the command on. (Why is a verse necessary for this?) Does it not follow (inductively)? viz.: There is plague-spot tumah and there is zav tumah. Just as plague-spot tumah does not obtain before the command, so zav tumah does not obtain before the command. (And, furthermore, it follows) a fortiori, viz.: If plague-spot tumah, where tumah obtains (even) through external causes, does not obtain before the command, then zav tumah, which does not obtain through external causes (viz. Vayikra 15:7 below) — how much more so should it not obtain before the command! — No, this may be so with plague-spots, whose tumah and taharah obtains only by (declaration of) a Cohein, as opposed to zavim, whose tumah and taharah obtains through (ascertainment of ) any man. This being the case, I would think that it obtains (even) before the command. It must, therefore, be written "if there be a flux" — from the command on.

3) "if there be a flux": I might think even if it flows from any place (i.e., from his nose or from his mouth), he becomes tamei. But it follows (that this is not so, for) there is tumah with a zav (a male) and tumah with a zavah (a female). Just as with a zavah, the place (the uterus) from which she becomes tamei with lesser tumah, (that of niddah, which does not require seven clean days or an offering), she becomes tamei with stringent tumah, (that of zivuth, which requires seven clean days and an offering) — so, with a zav, the place (the penis) from which he becomes tamei with lesser tumah (that of keri [an accidental seminal discharge]), he becomes tamei with stringent tumah (that of zivuth).