Niddah, Chapter 4, Mishnah 4. Our Mishnah presents a series of cases in which blood leaves a woman's body under circumstances that do not fit the standard category of dam niddah, and it records a dispute between Beis Shammai and Beis Hillel as to what level of tumah, if any, such blood carries.
Before entering the cases, one background principle is needed. Certain fluids that come out of a person's body, such as saliva and urine, are considered mashkin that convey tumah, but only while they are still moist. Once they have dried, they no longer transmit tumah at all. That distinction, between tamei when wet and clean when dry, is the yardstick our Mishnah uses again and again.
The blood of a nochris and the pure blood of a metzoras
The Mishnah begins with two cases stated side by side: "Dam nochris vedam taharah shel metzoras." The first concerns blood issuing from a nochris. The second concerns the dam taharah of a metzoras, a woman stricken with tzaraas who has also become a yoledes. After childbirth a woman passes first through yemei tumah, and afterward through a lengthy period of yemei taharah: thirty three days if her child was a boy, sixty six if her child was a girl. Blood that appears during those later days is dam tohar, pure blood. The Mishnah's question is how that rule works out when this very yoledes is afflicted with tzaraas as well.
"Beis Shammai metaharin." In the view of Beis Shammai, both of these are wholly tahor. Whatever issues from a nochris bears no tumah at all, and the pure blood of the days following childbirth retains its purity even in a woman who bears tzaraas.
"Uveis Hillel omrim kerukah uchemeimei ragleha." Beis Hillel disagree and assign this blood the status of her saliva and her urine. It is not dam niddah, but it is one of the fluids that leave her body, and therefore it conveys tumah while it is still moist and conveys nothing once it has dried.
The blood of a yoledes who has not yet immersed
"Dam yoledes shelo savlah." Here the case is a woman who gave birth, whose yemei tumah and yemei taharah have run their course, and who is obligated to immerse in a mikveh but has not yet done so. She now sees blood. What is its status?
The ruling of Beis Shammai here is "kerukah uchemeimei ragleha": this blood is grouped with the other liquids discharged from her body, following the pattern of spittle and urine. As long as it is still wet it is tamei, and once it has dried it imparts nothing.
"Uveis Hillel omrim metamei lach veyavesh." Beis Hillel hold that since she has not yet gone to the mikveh, this blood is genuinely tamei, and it conveys tumah whether it is found moist or whether it has already dried.
Where both schools agree: a yoledes bezov
Where both schools agree: a yoledes bezov
The Mishnah ends with a ruling on which the two schools stand together: "Umodim beyoledes bezov shehi metama lach veyavesh." The subject is a zavah at the time of her delivery. In the ordinary case, once a yoledes has finished the days of tumah, what follows, thirty three days for a male infant and sixty six when the child is female, are days of purity, and blood appearing then is dam tohar. A woman whose delivery came upon her while she stood in a state of zivah is different. When her days of tumah have passed, she is still bound to count shivah neki'im, seven clean days, in order to discharge her obligation as a zavah, and only from that point on does her blood carry the status of dam tohar.
Our case, then, is a woman who gave birth as a zavah and who has not yet counted her seven clean days, and blood emerges during that period. Here Beis Shammai concede to Beis Hillel: this blood is fully tamei, and it conveys tumah both when it is moist and when it is dry.