Niddah, Chapter 4, Mishnah 2. Before we open the words of the Mishnah, we need one basic halacha in place. When blood emerges from a woman's uterus, her age plays no role at all in the din. Even the tiniest infant girl, if blood comes from her, receives the status of a niddah, and whatever she touches, and whatever she lies or sits upon, becomes tamei mishum niddah.
For that reason, Klal Yisroel guarded this matter closely throughout the era when the Beis Hamikdash still stood. A father and mother kept watch over their little girls, and if blood was discovered, it was handled as dam niddah with all the gravity that halacha demands. Such vigilance was simply the accepted way of life among Yidden.
There were, however, two groups that did not maintain this standard: the Kussim and the Tzedukim. Since we cannot rely on their vigilance, we are forced to relate to their daughters mi'safek as though they are niddos already from infancy. That is the subject of our Mishnah and the one that follows it.
Niddos from the cradle
The Mishnah's opening words are "Bnos Kussim niddos me'arisasan". From the cradle onward, a Kussi girl is treated as a niddah. And who were the Kussim? They were gerim. They had come into Eretz Yisroel as gentiles, and what brought them to convert was sheer terror: lions were tearing into them, and that dread is what drove them toward Yiddishkeit. When a conversion is produced by fear instead of by a love of Torah, a question mark hangs over it permanently. Their attachment to Torah and mitzvos was therefore under suspicion, and in actual practice their observance was uneven.
Given that, we cannot assume that they were careful in hilchos niddah, watching their infant daughters and reacting properly to any blood that appeared. Therefore we treat the daughters of the Kussim as temei'os mishum niddah from the moment they lie in the cradle. It bears emphasizing that this is a tumah mi'safek, a tumah imposed on account of doubt.
The men of the Kussim
The Mishnah now turns to the men: "VehaKussim metamim mishkav tachton ke'elyon". Should a Kussi seat himself upon several coverings stacked one atop another, the lowest of them contracts tumah to precisely the same degree as the topmost one. That particular measure of tumah belongs to a man who has had relations with a niddah, and the Mishnah states plainly why it is applied here: "mipnei shehein bo'alei niddos". Since these men give no attention to hilchos niddah, we take it as a matter of course that each of them has been with his wife during her niddah.
A chumra that turns into a kula
The Mishnah records their custom next: "Vehein yoshvos", they sit, "al kol dam vadam", over every single sighting of blood. Any discharge at all is met with the laws of tumah, with no proper line drawn between blood that the Torah defines as dam niddah and blood that it does not. To the ear this sounds like nothing more than extra strictness, yet it can readily generate an actual leniency.
Consider a woman who sees some substance emerge and treats it as blood even though halachically it is not, and she immediately begins counting her seven days of tumah. Suppose that on the third or fourth day genuine blood appears. By the halacha, her seven days must be counted from that real sighting. But since she is already in the middle of a count that she began earlier, she will conclude her seven days too soon. What began as an extra measure of caution ends as an actual kula. The Kussim conduct themselves in exactly this fashion, and that is another reason we must treat them as temei'im mi'safek.
The consequences of a tumah of doubt
Since this tumah rests upon doubt rather than certainty, what follows from it is limited. "Ve'ein chayavim aleihem", one does not bear liability on their account, "al bi'as Mikdash": should a Yid become tamei by touching a Kussi, or by putting on a garment upon which a Kussi had been sitting, and he then enters the Beis Hamikdash, no korban is brought for that entry. "Ve'ein sorfim aleihem es haterumah": terumah that met with such tumah is not consigned to the flames. The Mishnah gives the reason in its own words, "mipnei shetumasam safek", their tumah being merely doubtful. Where tumah is only a possibility and never established, the grave outcomes that trail a definite tumah simply do not take effect.