As we mentioned in the previous Mishnah, by strict law the laws of the tumah of a zav and a niddah apply only while they are alive. Once they die, by Torah law they are no longer considered impure on account of their zivah or niddah - although they are in any case impure with the most severe form of tumah of all, the tumah of the corpse itself.
A woman who died and blood came out of her:
If a woman died, and after her death a revi'is of a log of blood, which is a certain volume of blood, came out of her womb - this blood is not impure under the law of the blood of a niddah, since at the moment it emerged she was no longer considered a niddah. Nevertheless, the Mishnah rules that it is "tamei mishum kesem" - impure as a source of tumah. The precise wording of the Mishnah teaches that its tumah does not stem from its being blood that came out of a niddah, for the blood of a niddah is impure by the very fact that it emerged from a niddah, whereas here the blood is not considered the blood of a niddah.
A halachah of Moshe from Sinai:
According to the Mishnah, the womb itself is considered a source of tumah, and we have a special tradition going back to Moshe Rabbeinu, who received it at Mount Sinai: blood that touched the womb - and blood that comes out of it has certainly touched it - is impure at the very same level as the womb itself. Just as the womb is an av hatumah, so too the blood that comes out of it is an av hatumah. This does not follow the ordinary rules of tumah, according to which whatever becomes impure takes on a lower level than its source of tumah; rather, this is one of the exceptions established by the halachah of Moshe from Sinai.
For this reason, even though the blood did not come out of a woman who has the status of a niddah, it is still impure at the level of an av hatumah simply because it came from the womb. And just as blood that comes out of a niddah is a source of tumah in any quantity at all, even the tiniest amount, so too this blood that comes from the womb.
A revi'is of a log and the tumah of an ohel:
The Mishnah adds that since there is here a revi'is of a log of blood, which is a significant quantity, it also conveys tumah through an ohel: anything found together with it under one roof becomes impure, because it is considered part of the body of the deceased. The tumah of a corpse passes through an ohel, but only if there is a revi'is of a log; whereas for tumah through contact or through carrying, even the smallest amount suffices.
The view of Rabbi Yehudah:
Rabbi Yehudah disagrees and holds that the blood does not become a source of tumah on account of its coming from the womb, "mipnei shene'ekar mishemeisah" - because it was dislodged after she died, that is, it emerged from the womb only after her death, and she is no longer considered a niddah. Rabbi Yehudah disputes the very existence of a halachah from Sinai that rules otherwise, and in his view the only tumah that applies here is on account of its being part of the body of the deceased; therefore only if it contains a revi'is of a log will it be a source of tumah.
"Umodeh Rabbi Yehudah beyoshoves al hamashber":
Rabbi Yehudah concedes in the case of a woman who was sitting on the birthing stone, which is the mashber, and died while giving birth, and a revi'is of blood came out of her womb - that in this case the blood is impure as a kesem, meaning that it becomes a source of tumah by virtue of its coming from the womb. The reason: we assume that at least some of the blood came out of her body while she was still alive. Such blood is called 'dam tevusah' - blood, part of which came out of the person while alive and part after death. Since there is a doubt whether most of the blood came out while she was still alive, and at the very least it is possible that this was the case, we impose upon it both stringencies: the stringency of blood that came out after death, and also the stringency of the blood of a niddah, which is a source of tumah even in less than a revi'is of a log.
The View of Rabbi Yosei:
Rabbi Yosei disagrees and says: "lefichach ein hadam metamei b'ohel" - it is not a source of tumah that transmits tumah through a tent, and whatever is under the same roof with it does not become tamei. In his opinion we assume that most of the blood left her body before she died, and therefore its status is that of ordinary blood of a niddah, whose tumah does not pass to items found with it under the same roof; only one who touches it or carries it becomes tamei.
In summary: two separate stringencies stand opposite one another:
Blood of a niddah: its stringency is that even the smallest quantity of it is a source of tumah.
Blood from the body of a corpse: its stringency is that it transmits its tumah to everything under the same roof with it, but only if it amounts to at least a revi'is of a log.