Zavim, Chapter 5, Mishnah 7. The mishnayos here in the latter half of the perek are busy calibrating the reach of each tumah: how many stages of contamination a person carries along with him, and where that chain gives out so that nothing remains but the power to disqualify terumah. Our mishnah brings that same reckoning to the various discharges of a zav and to niddah blood.
The mishnah begins: "HaNogea b'zovo shel zav uv'ruko uv'shichvas zar'o uv'mey raglav uv'dam haniddah", one who comes into contact with the zov of a zav, with his spittle, with his semen, with his urine, and with the dam of a niddah. Every one of these carries a single ruling: "metamei shnayim u'posel echad", he is metamei two and posel one.
The ma'ayanos haZav
These discharges go by the name ma'ayanos haZav, the "wellsprings" of the zav. Halachically they rank as an av hatumah, and an av hatumah carries the strength of metamei adam l'tamei begadim: it renders a human being tamei to a degree that he himself then conveys tumah to garments. So a person in contact with any of them has not only taken on tumah for himself; for as long as that contact continues, the clothing on his body is drawn in as well.
Metamei shnayim u'posel echad
That phrase matches what the earlier mishnayos taught about an av hatumah, and here is how the count runs. His garments, together with any kli he lays a hand on, become a rishon l'tumah. Such a kli turns food it touches into a sheini l'tumah. Food at that level, being a sheini, does not make terumah tamei but merely disqualifies it, leaving it a shlishi. Two genuine grades of tumah are therefore generated, a rishon and a sheini, followed by one stage that is disqualification alone, the shlishi in terumah. Hence: two contaminated, one invalidated.
Peirash: once he lets go
The mishnah goes on: "peirash, metamei echad u'posel echad", having withdrawn from them, he is metamei one and posel one. The instant the contact is broken, his capacity to render garments tamei falls away, and he stands in the plain position of anyone who has touched an av hatumah, a rishon l'tumah. In that capacity he makes food a sheini l'tumah, and food of that grade proceeds to disqualify any terumah it meets. One contaminated and one invalidated, precisely as the mishnah before ours laid out.
Touching, moving and carrying
Next the mishnah states: "echad hanogea v'echad hameisi", contact and movement are equivalent here; whether he touched the fluids or shifted them, the din does not change. Left outside the rule is bearing alone: a person who does no more than support the ma'ayanos haZav, without budging them at all, remains tahor.
A minority opinion closes the mishnah: "Rebbe Eliezer omer af hanosei", one who bears them belongs in the list too. The Gemara, though, reworks the formulation into "v'hu shenosei": the point is not that a bearer is simply appended to the cases of touching and moving, but that bearing without any shifting is itself reckoned like contact, and this only in one particular configuration, when the man is positioned below and the fluids rest above him. Under those conditions alone would Rebbe Eliezer declare him tamei and capable of conveying tumah. Halachah does not adopt his view.
Our mishnah therefore lays out the ma'ayanos haZav in full: an av hatumah in every sense, conveying tumah by contact and by movement alike, generating two grades of tumah plus a third of disqualification for as long as the contact endures, and receding to the standing of a simple rishon l'tumah as soon as that contact is broken.