Niddah, Chapter 7, Mishnah 5. This chapter deals with kesamim, bloodstains found on a garment whose owner and origin are unknown. The recurring question is whether we must treat such a stain as blood of a niddah, which carries tumah, or whether we may attribute it to some other source that involves no tumah. Our mishnah addresses stains according to the place where they turn up, and it closes with a dispute about the buildings the Kusim set aside for this purpose.
The mishnah opens: "Kol haksamim hanimtza'im bechol makom tehorim." Stained garments found anywhere at all are tahor. Even in cities inhabited by Jewish women, a stained garment discovered lying about is not treated as a source of tumah. The reason rests on the care Jewish women took with such garments: a woman who found a stain would put the garment away rather than leave it in circulation. Since this garment was not put away, the blood on it presumably came from some other source entirely, and it is not the blood of tumas niddah.
The mishnah then carves out an exception: "chutz min hanimtza'im bachadarim," except for stains found in the chambers. These were the rooms in which women would specifically stay during the days when they were temeios. A stained garment found there cannot be pushed off onto another explanation, since that is exactly the place where such a stain is to be expected. The mishnah adds "uvisvivos beis hatumah," and in the surroundings of the house of tumah, meaning the general area around those rooms or houses where the women remained while they were temeios. Stains found there are likewise treated as tamei.
From here the mishnah raises the level of concern further: "Beis hatumos shel Kusim metam'in ba'ohel." The buildings that the Kusim designated for this purpose present a problem that goes well past the tumah of niddah blood. They transmit tumah by way of ohel, the tumah that spreads under a shared covering, and the mishnah supplies the reason: they were in the habit of interring their nefalim in these very structures, "kovrim sham es hanefalim." It happened at times that a woman staying in such a room lost her pregnancy, whether through a stillbirth or a miscarriage, and the Kusim would place the fetus in the ground right there, in the building she had been using. The result is that someone who steps inside faces two separate worries: the tumah attached to a stain, and, beyond it, the tumah of a structure that has a corpse resting inside it, which renders tamei whatever is found beneath its roof.
Rabbi Yehuda holds that one need not go so far: "lo hayu kovrim, ela mashlichin." The Kusim did not actually bury the nefalim in these places; they simply cast them out, "vechayah gorartan," and wild animals would drag them off and consume them. Precisely because the Kusim were so lax and never gave the fetuses a proper burial, nothing remains buried in those buildings. What sounds at first like a criticism of their conduct turns out to yield a leniency: according to Rabbi Yehuda one need not be concerned for tumas ohel in those structures at all.
The mishnah thus moves along a scale. Where a stain can be attributed elsewhere, it is tahor; where the place itself points to niddah, the stain is tamei; and where a place may hold a body, the concern rises to the tumah of an ohel, unless, as Rabbi Yehuda maintains, no body was ever left there to begin with.