Keilim, Chapter 1, Mishnah 3. This chapter is constructing a ladder of tumah, ascending step by step from the lighter avot hatumah toward the heavier, and each rung is measured by a single test: how broad is this source's reach, and through how many channels can it pass its tumah onward? Here the ascent brings us into the realm of tumah that comes from the human body: the niddah, the zavah, the zav, and the boel niddah. Before the words of the mishnah can be read properly, these categories have to be understood.
The niddah
Once blood emerges from the womb, a woman holds the status of niddah, and her tumah runs for seven days at the very least. Mid'Torah, when those seven days have gone by, immersion in a mikveh restores her taharah. How many of the days actually brought a sighting of blood is irrelevant to this calculation: one day of bleeding or six, either way the seventh is the day on which the Torah has her immerse. As for a man who is intimate with her, the boel niddah, he is liable to karet.
The zavah
Following the days of the niddah cycle come another eleven days, known as the yemei zivah. Should blood appear on a single one of them, or on two, her status is that of a zavah ketanah, a minor zavah. She is temeah, and to demonstrate that the flow has ceased she keeps one clean day matching the day of the sighting, yom keneged yom. Immersion in a mikveh follows, her taharah is restored, and she may resume married life.
Sightings on three days running inside that eleven-day period change matters entirely: her status is now that of a zavah gedolah, a major zavah. From that point she counts a full seven days clean of any blood. Immersion takes place on the seventh day itself, the last of the count, and once night falls she is tehorah and married life resumes.
To this Torah framework the chachamim added humrot. Any discharge of blood is now treated automatically as if it created the status of a zavah gedolah. That is why every ordinary niddah today counts seven clean days, and why her immersion takes place only at night.
The zav and the boel niddah
The zav is one who suffers a discharge of an abnormal sort, a discharge that comes from illness, and who has two such sightings within a period of two days. Ordinary mikveh water will not purify him: his immersion has to take place in a spring of flowing water.
One who is intimate with a niddah, or with a zavah, carries tumah for seven days, and immersion in a mikveh at their conclusion restores his taharah. Throughout that period he transmits tumah along the two channels familiar to us from the neveilah and from the mei chatat, the purification waters, of the preceding mishnah: maga, which is contact, and massa, which is carrying. What our mishnah supplies is one additional route of transmission unique to him, and it is exactly that addition which lifts him to a higher rung.
The boel niddah and the mishkav beneath him
The mishnah begins: "Lemalah mehen bo'el niddah" - above every source enumerated so far, in the scope of his power to transmit, stands the boel niddah, one who has been intimate with a niddah, "shehu metamei mishkav tachton ka'elyon," because whatever lies underneath him takes on tumah in the manner of the underlying layers of a zav's bedding, meaning to the same level as the layer resting immediately above.
Picture him lying on a bed. That bed is a mishkav, and he rests his body weight upon it. Whether it is a single bed, a mattress, or a stack of beds and mattresses piled one upon the other, they all become a rishon letumah. As a rishon they are able to render food and drink tameh, but they do not render kelim or people tameh. In this respect he is more stringent than the neveilah and the purification waters, neither of which conveys tumah to what a person is sitting or lying upon.
The discharges themselves
The mishnah climbs again: "Lemalah mehen zovo shel zav veruko veshichvat zaro umeimei raglav vedam haniddah, shehen metam'in bemaga uvemassa" - higher still are the discharge of the zav, his spittle, his semen and his urine, and the blood of a niddah, for these convey tumah both through contact and through carrying.
The difference is a fine one. In the previous mishnah we learned that carrying a neveilah or the mei chatat brings tumah to a person's garments, whereas touching them leaves those garments tahor. In the case before us, mere contact with a zav's zov, or with dam niddah, already suffices to render the garments on that person tameh.
Put another way: if a person is holding a keli in one hand and touching dam niddah with the other, the dam niddah renders him tameh, and it also renders that keli tameh through him. This is where these substances are stricter than the sources listed earlier, which pass tumah on to a third object only when they are being carried.
The merkav
"Lemalah mehen merkav" - higher than all of these is a merkav, "shehu metamei tachat even mesama," for it conveys tumah even from beneath a placed stone. A merkav, as we know, is an object designated for riding, such as the handle of a saddle. The saddle itself is not our subject; that belongs to the category of moshav, objects made for sitting.
What is the case of the placed stone? A person rests his weight upon a stone, and beneath that stone lies the merkav. Earlier we spoke of a person resting directly upon an object, with everything beneath it becoming tameh. Here the mishnah teaches the reverse arrangement: the tameh object lies below, a stone sits above it, and the person who rests his weight on the stone becomes tameh. So the merkav does not only convey tumah through being carried or touched; it reaches upward to whoever presses his weight down upon it.
In truth the stone need not have been mentioned at all, since the ruling would hold in any case. It is mentioned in order to teach that a merkav conveys its tumah even through an intermediary that is itself incapable of receiving tumah. A stone cannot become tameh, and still the tumah passes through it.
The mishkav
"Lemalah min hamerkav mishkav sheshaveh maga'o lemassa'o" - higher than the merkav is the mishkav, the object a person lies upon, for its contact is equal to its carrying: it conveys tumah through touch exactly as it does through being carried.
Compare the two. One who carries a merkav conveys tumah to garments or other kelim he is touching at that same moment. But one who merely touches a merkav does not convey tumah onward to those garments or kelim. With a mishkav there is no such difference: even one who only touches it conveys tumah to the garments and kelim he is in contact with at the same time.
The zav himself
"Lemalah min hamishkav hazav" - and higher than the mishkav is the zav himself. Like a mishkav, the zav conveys tumah from beneath a placed stone. But he has something the mishkav does not: "shehazav oseh mishkav ve'ein mishkav oseh mishkav," the zav creates a mishkav, whereas a mishkav does not create a mishkav.
The moment the zav sets his weight down on bedding, whether a bed or a mattress, he transforms it into an av hatumah, and every layer piled below it becomes an av hatumah too. Contrast a keli that touches an item bearing the tumah of a mishkav: that keli rises no higher than the status of a rishon. Here lies the closing rung of the mishnah: the zav is a source with the strength to generate further sources, and by virtue of that strength he ranks even above the mishkav whose tumah came from him in the first place.