Tevul Yom, Chapter 2, Mishnah 2. This perek is occupied with the status of liquids in the laws of tumah: fluids that emerge from a person's body, and fluids that a tamei person touches. The perek opened by teaching that with a tevul yom, one who has immersed and awaits only nightfall, the liquids that leave him and the liquids he touches are equally unable to convey tumah to anything further. Our mishnah now turns to everyone else who is tamei and asks what the rule is for them.
The Mishnah teaches: "usha'ar kol hatemei'im, bein kalim bein chamurim," all other people who are tamei, whether their tumah is of a light degree or of a stringent degree, "hamashkim hayotz'im mehen kemashkin shehu nogei'a bahen." Here too the two cases are equated: the liquids that come out of such a person's body are given the same status as the liquids that he touches with his body.
Yet the status they share is not the leniency enjoyed by the tevul yom. As the Mishnah words it, "eilu va'eilu techilah": whatever fluid issues from his body and whatever liquid his body comes into contact with both carry the level called techilah, meaning a rishon letumah, the opening rung on the ladder of tumah. Liquid at this level is truly tamei, and it retains the power to pass that tumah on to whatever it meets.
One case, however, is set apart, and the Mishnah flags it: "chutz min hamashkeh shehu av hatumah." A single category of fluid rises past the rung of a rishon and occupies a far more severe standing altogether, that of an av hatumah. This describes the discharges of a zav, upon which the Torah itself confers that grave status directly. Once this exception has been set aside, whatever fluid issues from a person who is tamei carries the level of a rishon letumah and nothing beyond it, exactly matching the liquids that come into contact with him.