Midrash su Levitico 15:22

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.

12) (Vayikra 15:22) "And whoever touches any object that she sits upon shall wash his clothes": Just as below (i.e., Vayikra 15 verse 21, which elucidates verse 20), she confers tumah only upon a mishkav that is distinctive for lying upon and a moshav that is distinctive for sitting upon, here, too, she confers tumah only upon a mishkav that is distinctive for lying upon and a moshav that is distinctive for sitting upon.

13) I might think that as far as conferring tumah is concerned, she confers tumah only upon a mishkav that is distinctive for lying upon and upon a moshav that is distinctive for sitting upon; but as far as becoming tamei is concerned, all kinds (of moshav and mishkav, even non-distinctive) are susceptible of this. For there are many things which become tamei and yet do not confer tumah (upon other things). It is, therefore written "upon it" - "upon it." "Upon it" is written in respect to becoming tamei (Vayikra 15 verse 22, lit., "that she sits upon it"), and "upon it" is written in respect to conferring tumah (Vayikra 15 verse 20). Just as with "upon it" in respect to conferring tumah, only a mishkav distinctive for lying upon and a mishkav distinctive for sitting upon confer tumah, so with "upon it" in respect to becoming tamei, only a mishkav distinctive for lying upon and a mishkav distinctive for sitting upon become tamei.