Zavim, Chapter 5, Mishnah 10. The closing portion of this chapter surveys a range of tumah categories and compares them to one another, and this Mishnah picks up directly where the previous one left off, measuring one kind of tamei person against another so that we know exactly how severe each one is.
The Mishnah opens: "Ba'al keri k'maga sheretz." A man who has had a seminal emission carries the identical status as a person who has touched a sheretz. Such a man becomes tamei through his emission (he is called a ba'al keri), and his halachos turn out to be precisely those of one who made contact with a sheretz: both of them are a rishon l'tumah.
Why does the Tanna reach for this particular comparison? Because the link between the ba'al keri and the one who touched a sheretz is itself derived from a hekesh, a scriptural juxtaposition brought in the Gemara in Shabbos. And there is something instructive in the pairing. The Mishnah is telling us not only whom the ba'al keri resembles, but whom he does not resemble. One might have assumed that a ba'al keri belongs alongside a zav, since a discharge from the body is involved in both cases. In truth, though, the two are only superficially similar; they do not share the same body of law. The ba'al keri's true match is the person who touched a sheretz.
The next clause reads: "U'vo'el niddah k'tmei meis." A man who has been intimate with his wife during her niddah state is placed in the very same category as someone who became tamei through a corpse. Each of the two is an av hatumah. The practical consequence is the seven day minimum: exactly as tumas meis holds a person for a full week, so the bo'el niddah counts seven days before he can go down to a mikveh and emerge tahor.
The parallel, though, is not exact, and the Tanna himself flags the difference: "Ela shechamur mimenu bo'el niddah" - in one respect the bo'el niddah is the stricter of the pair. He is "metamei mishkav umoshav": whatever he lies down upon or sits upon becomes tamei from him, which is not the case with someone who touched a corpse. The Mishnah then defines the reach of that transmission as "tumah kalah," sufficient "l'tamei ochlin umashkin," to pass tumah along to food and to drink.
Those two words, "tumah kalah," are doing real work. When a zav lies or sits on something, that object itself becomes an av hatumah. Bedding and seating touched in this way by a bo'el niddah never climb that high; they stop at the level of a rishon l'tumah. The result is that such items can indeed pass tumah on to ochlin and mashkin, but a person or a vessel that comes into contact with them remains tahor.
So this Mishnah leaves us with two clean equations and one careful adjustment: the ba'al keri is measured by the sheretz, the bo'el niddah is measured by contact with a corpse, and the bo'el niddah is a shade more severe, because his bedding and seat carry tumah, though only in its lighter form.