Niddah, Chapter 10, Mishnah 6. The Mishnah before us takes up a striking situation: a woman has passed away, and afterward blood flows out of her body. Which tumah does such blood carry: the tumah of a niddah, corpse tumah, both at once, or none at all?
The Mishnah begins by laying out the case. "Ha'isha shemeisa", a woman who has died, "veyatza mimena revi'is dam", and a revi'is of blood was discharged from her. A revi'is is a liquid measure of roughly three and a half ounces, and that amount is not incidental here: a revi'is is exactly the quantity of blood from a dead body that carries corpse tumah.
The ruling of the first opinion
Of such blood the Mishnah says: "mitama mishum kesem umitama b'ohel". The blood carries two separate kinds of tumah at once. First, it is treated as a kesem, a bloodstain, and therefore conveys tumas niddah. Second, since a full revi'is emerged, it conveys tumas ohel, the tumah of a corpse that fills a tent or covered space, just as a significant portion of a dead body would.
Rabbi Yehuda: no tumas kesem
"Rabi Yehuda omer". Rabbi Yehuda's ruling strikes out the first of those two tumos. "Eina mitama mishum kesem", the blood does not render anything tamei in the way a bloodstain would, "mipnei shene'ekar mishemeisa", because it was loosened from its place inside her only once life had already left her. For blood to acquire the status of dam niddah, it has to exit a woman who is alive and in a state of niddah. Blood that detached at the moment of death, or later still, never entered that category at all.
Where Rabbi Yehuda agrees
The Mishnah then records one situation in which Rabbi Yehuda himself grants the point: "umodeh Rabi Yehuda". The case is "beyosheves al hamashber umeisa", a woman sitting on the mashber, the birthing chair, whose labor had already begun, and who died there, "veyatza mimena revi'is dam", and a revi'is of blood emerged from her, "shehi mitama mishum kesem", and here the blood does convey the tumah of a kesem. Since labor had already set in before she died, we work with the assumption that this blood had begun making its way out during her lifetime, and blood that departs from a living woman is true dam niddah.
Rabbi Yose: then not tumas ohel
"Omer Rabi Yose". Rabbi Yose carries that same assumption through to its logical end: "lefikach eina mitama b'ohel", and therefore this blood does not spread tumah through a tent. If we have concluded that the blood left her before death took hold, it is not the blood of a corpse, and tumas ohel is generated only by corpse blood, never by blood that issued from someone still living. So in the case of the woman on the birthing chair, the blood counts as a kesem, yet it does not fill an enclosed space with corpse tumah.
The halacha
The halacha follows neither Rabbi Yehuda nor Rabbi Yose, but the tana kama, the first opinion of the Mishnah. Blood that emerges from a woman who has died is treated stringently in both directions: it conveys the tumah of a kesem, the tumah of niddah, and when it amounts to a revi'is it also conveys tumas ohel as blood of a corpse.