Zavim, Chapter 5, Mishnah 1. The fifth perek opens by gathering together, in clear and orderly fashion, a number of principles of tumah and taharah that have already come up in the earlier mishnayos. It makes an excellent review before moving forward.
Touching and Moving a Zav
Four cases open the Mishnah: "hanogea bazav", someone who touches a zav; "o shehazav nogea bo", or a zav who does the touching; "hameisit es hazav", someone who shifts a zav from his place; "o shehazav mesito", or a zav who shifts him. About all four alike the ruling is that this person becomes "metamei ochlin umashkin", and "uchlei shetef" as well, yet only "bemaga, aval lo bemassa".
Such a person conveys tumah to foods (ochlin), to liquids (mashkin), and to kelei shetef, meaning vessels that can be restored to taharah by rinsing in a mikveh. But he conveys this tumah only through maga, actual contact, and not through massa, carrying an object without touching it.
The Principle of Rabbi Yehoshua
At this point the Mishnah sets down a sweeping rule in the name of a Tanna: "Klal amar Rabbi Yehoshua", and here is his principle: "kol hametamei begadim bishas maga'o, metamei ochlin umashkin lihyos techilah, v'yadayim lihyos sheniyos, v'eino metamei lo adam v'lo klei cheres."
Whom does Rabbi Yehoshua have in mind? Anyone whom the Torah gives the strength to make begadim, garments, tamei, and specifically during the moment that he is still joined to the av hatumah itself, such as a zav or a zavah whom he is touching right then. At that very moment he can also raise ochlin and mashkin to the first rung of tumah, rishon letumah, which our Mishnah labels techilah. And should he touch someone else's hands, those hands go down one rung further and become sheini letumah.
What he cannot do, even at that moment, is render a person tamei, and he cannot render a klei cheres, an earthenware vessel, tamei. He affects garments but not people.
After He Separates From the Source
The Mishnah continues: "l'achar perishaso mimetam'av, metamei mashkin lihyos techilah, v'ochlin v'yadayim lihyos sheniyos, v'eino metamei begadim."
Once he is no longer in contact with the item that rendered him tamei, his power drops somewhat. He can still bring liquids to the level of techilah, rishon letumah, and he still brings foods, as well as hands, to the level of sheini letumah. This is because he himself now carries the status of a rishon letumah.
What he can no longer do is affect begadim, and likewise he cannot affect keilim at all. The reason is straightforward: a rishon letumah has the ability to convey tumah only to ochlin and mashkin, to foods and liquids, but not to vessels.
In these few lines the Mishnah has laid out the whole picture: what a person becomes while he is in contact with a zav, what he becomes once he has separated, and precisely which categories of objects each stage can affect.