Zavim, Chapter 5, Mishnah 11. Our masechta closes with a list, and the list has a common thread: people and objects that cannot actually transmit tumah, yet were nevertheless given the power to render terumah unfit.
The opening words are "Eilu poslin es haterumah": the following render terumah unfit. Measured by Torah law, nearly nothing on this roster would disqualify terumah; what stands behind each case is a takanah of the Chachamim. Maseches Shabbos preserves the story: the Sages of that generation convened in Chananya ben Chizkiya ben Garon's attic and issued a whole series of decrees, and several of the cases listed right here belong to that session. Only one entry, the tevul yom, is Torah law, and we will address it when we reach it. So the Tanna rounds off the perek by assembling those people and items that have no real ability to spread tumah at all, yet which the Sages ruled would ruin terumah the moment they touch it, each ruling driven by its own concern.
Eating food with the status of a rishon
"Ha'ochel ochel rishon." The opening case concerns someone who eats food that carries the level of rishon l'tumah. A food arrives at that level when an av hatumah touches it, say food touched by a sheretz. Whoever eats such food takes on the level of sheni l'tumah, with the result that terumah coming into his hands is spoiled. Here too the source of the ruling is the Sages, not the Torah.
What moved the Chachamim to legislate here? For most of the items in this list one and the same concern is at work: the worry that a Kohen would eat terumah while tamei food or drink was still in his mouth, so that the terumah itself would become tamei and the Kohen would be eating tamei terumah, a serious problem. To keep him at a distance from that scenario, the Chachamim declared the terumah unfit in advance, so that the Kohen would never so much as pick it up, since it already carried a status of tumah.
Eating food with the status of a sheni
"Ha'ochel ochel sheni." The Mishnah turns to one who consumes food carrying the level of sheni l'tumah. A food arrives at that level after a rishon l'tumah touches it, as when a person who had handled a sheretz then handled the food. Whoever eats such food becomes a sheni himself, and terumah that he handles is likewise ruined.
One who drinks tamei liquids
"V'hashoseh mashkin t'meyin." A person who drinks liquids that are tamei likewise becomes a sheni l'tumah and invalidates terumah. All three of these cases rest on the same fear: the Chachamim did not want a Kohen coming to eat terumah that his own mouth would render tamei, and so they issued the gezeirah.
Drawn water
"V'haba rosho v'rubo b'mayim sheuvin." This refers to a person who entered drawn water with his head and most of his body. What was the setting? It was standard practice, once a person came up from the mikveh, to rinse off in mayim sheuvin, drawn water, much as one would take a shower afterward. The motive was understandable: mikveh water came from caves, cloudy and none too inviting, and a person emerging from it wanted to get clean. Whoever has used a mikveh in our own times that was not exactly sparkling can sympathize.
The problem was what this practice did to people's thinking. In the times of Chazal people came to imagine that it was the drawn water, and not the mikveh, that had purified them. Of course taharah has nothing whatsoever to do with clean or dirty; it rests entirely on what the Torah established as purifying. The unappealing cave water is exactly what purifies a person. But the public grew confused and reasoned that such water could not possibly be the source of taharah, and that it must be the fresh drawn water they rinsed with that did the work. To stop people from relying on drawn water, the Chachamim decreed that such a person is tamei.
The decree was stretched even wider, as the Mishnah states: a person who is completely tahor, should "shelosha lugin mayim sheuvin", three lugin of drawn water, come down upon his head and most of his body, already disqualifies terumah he touches, and he stays that way until he immerses in a valid mikveh.
The scroll
"V'hasefer." Which scroll does the Mishnah mean? In the era of Chazal it was common practice to keep terumah produce in the same place as Torah scrolls, on the reasoning that both are sacred and both deserve one secure spot. What actually happened was that mice were drawn to the food, got into those hiding places, and chewed the scrolls to ruin. So the Sages decreed that a scroll disqualifies terumah when the two touch, precisely in order that people stop storing terumah next to their sifrei kodesh. The word covers not only a Sefer Torah but any of the sacred writings of Tanach. A sefer is treated on the level of a sheni l'tumah, so terumah it comes into contact with is unfit. Once that was the halachah, no one would gamble with his terumah by leaving it beside his sifrei Tanach, and the scrolls were safe.
Hands
"V'hayadayim." The reference is to hands that were not rinsed. Should a person handle terumah before washing, he ruins it. Why? Because hands roam: there is no telling where they have been, and people simply do not keep track of everything they touch. The Sages therefore assigned stam yadayim, the unwashed hands of an ordinary person, the level of a sheni l'tumah, which spoils terumah on contact. And the remedy? Washing. With netilas yadayim the hands hold on to their taharah, so long as one then takes care not to touch anything that would render them tamei again.
Tevul yom
The list continues with the tevul yom. This is a person who became tamei, whether from an av hatumah or from a rishon l'tumah, and who has already immersed in a mikveh, but immersion by itself is not the end of the process: he must wait for the sun to set in order to be fully tahor. Accordingly, someone who immersed at two in the afternoon and touches terumah at any point before nightfall invalidates it. This waiting requirement is what is called ha'arev shemesh, the arrival of nightfall. This item, unlike the others, did not originate in the assembly in the attic of Chananya ben Chizkiya. It is derived from a pasuk in Chumash and is a Torah law, while the remaining cases were added by that enactment of the Sages.
Food and vessels made tamei by liquids
The Mishnah's final clause reads "v'ha'ochlin v'hakeilim shenitmeu b'mashkim." Foods rendered tamei by liquids that were themselves tamei are given the level of sheni l'tumah and spoil terumah they touch. Behind this lies a wider safeguard concerning liquids, which pick up tumah with great ease, from a sheretz for instance; min haTorah such liquids are a rishon l'tumah. The Chachamim then legislated that liquids would transmit tumah in every case, even when nothing more than unwashed hands had touched them, and they carry the level of a rishon; food made tamei by liquids of that kind disqualifies terumah.
And the second half of the phrase, "v'hakeilim shenitmeu b'mashkim": keilim rendered tamei through tamei liquids. A utensil that a tamei liquid touched also receives the level of sheni l'tumah, so it too spoils terumah. Notice that the Sages legislated in two stages. Stage one: every tamei liquid is granted the level of a rishon l'tumah. Stage two: that rabbinically created rishon was given greater reach than the Torah's own rishon, transmitting tumah not merely to foods but to keilim as well.
Why extend it so far? For the sake of uniformity in the laws of tamei liquids. Strictly from the Torah it is only the ma'ayanos hazav, the bodily secretions of a zav discussed earlier in this chapter, that carry this kind of power. The Chachamim wanted all liquids treated alike, so that people would not grow confused, and so that they would not become lax with keilim that had touched the ma'ayanos hazav. They therefore ruled that vessels become tamei from contact with any tamei liquid, and not only from the ma'ayanos hazav taught in Mishnah 7.
And with this list, Maseches Zavim comes to a close. May your continued learning be blessed with much success.