The fourth chapter of Masechet Nida, known as Benot Kutim, begins with the Mishnah stating that the daughters of the Kutim are considered Nidot from birth. The Kutim were descendants from a place called Kuta, who converted to Judaism out of fear. There is a question regarding the legitimacy of their conversion, as they did not fully accept the traditions of the Sages, particularly the Oral Torah. Consequently, they did not observe certain halachot, including the principle that a girl could become a Nida and transmit ritual impurity even at a young age. Therefore, they were not careful with their young daughters regarding the laws of Tum'a and and Taharah if they saw blood.
The Mishnah further explains that the daughters of the Kutim are automatically considered Nida. This means that if a Zav (a man who has a seminal emission) sits on something, it becomes a Moshav, and if he lies on something, it becomes a Mishkav. Both the Mishkav and Moshav become a primary source of impurity, an Av Tum'ah, just like the Zav himself. However, when it came to the Kutim, their Mishkav (what they lay on) was treated with the same level of impurity as an Elyon, which is something carried by a Zav, making it a secondary source of impurity (Rishon LeTum'ah). The Gemara asks why such a ruling would be made, given that the Kutim do not observe the laws of Nida.
The Gemara, citing Rav Huna, explains that this decree was made because the Kutim would have relations with their wives who were Nidot. If a man has relations with a Nida, he also becomes ritually impure. The Gemara continues, stating that the Kutim would "sit on any blood," meaning they considered any blood discharge from their wives to be Nida blood, regardless of its color. This differs from the Jewish practice, where only specific colors of blood render a woman a Nida. For example, if a Kuti woman saw green blood on Monday and red blood on Tuesday, she would begin counting her seven days of Nida from Monday. As a result, she would immerse in the Mikveh seven days after Monday, even though she only became a Nida according to Jewish law on Tuesday. This premature immersion meant her husband would have relations with her while she was still a Nida according to Jewish law. Consequently, the Sages decreed that anything the Kutim sat on would become a Rishon LeTum'ah. While entering the Beit HaMikdash with impurity from a primary source (an Av Tum'ah) incurs a sin offering, impurity from a secondary source (a Rishon LeTum'ah) does not. This distinction was made because the Sages were not certain that Kuti men always had relations with Nidot, so they did not impose the full stringency of an Av Tum'ah.
The Mishnah's ruling that daughters of the Kutim are considered Nidot from birth raises a question: why single out Kuti girls? Jewish girls can also become Nidot at a young age if they see blood. The distinction lies in the fact that we are not sure if Kuti girls saw blood, and we do not investigate. This is because there is a minority of young girls who do see blood, and the Sages decreed that we must suspect this minority. The Gemara then asks which Rabbi holds that we must be concerned for a minority, as Jewish law typically follows the majority. The answer is Rabbi Meir.
The Gemara supports this by citing a Baraita concerning the laws of Yibum and Halitza. If a man dies childless, his brother must either perform Yibum (marry the widow) or Halitza (a ceremony releasing them from the obligation). If the brother is a minor (Katan), he cannot perform Halitza because the Torah uses the term "Ish" (man), which implies an adult. However, the Baraita also states that a minor cannot perform Yibum. The reason given is that a Katan might grow up to be sterile (a Saris), and the purpose of Yibum is to perpetuate the family. Similarly, a minor girl (Ketana) might grow up to be infertile (an Aylonit). Since Yibum is a Mitzvah that overrides the prohibition of marrying a sister-in-law, if it cannot fulfill its purpose, the prohibition remains. Most children do not grow up to be sterile or infertile, yet Rabbi Meir is concerned for this minority. This demonstrates Rabbi Meir's principle of being concerned for a minority.
The Gemara then challenges this comparison: the minority of children who become sterile or infertile is a "prevalent minority" (Mi'uta DeShchiach), meaning it occurs somewhat regularly. However, for a one-day-old girl to see Nida blood is a "non-prevalent minority" (Mi'uta DeLo Shchiach), which is extremely rare. Even Rabbi Meir might not be concerned for such a rare occurrence. The Gemara refutes this, arguing that it is indeed a prevalent minority. It cites a Baraita by Rabbi Yehoshua, which recounts three incidents: one in Ein Bold, one in Beit Choram, and one in Pumbedita, where infant girls saw Nida blood and were immersed in the Mikveh even before their mothers. These stories demonstrate that such occurrences, while a minority, are not so rare as to be dismissed.
The Gemara then asks why the girl in Pumbedita (Babylon) was sent to the Mikveh, as there were no Terumah (sacred food) issues there, unlike in Eretz Yisrael. In Babylon, the Sages only prohibited eating Terumah when one was impure from a bodily discharge like Nida or Zavah. Furthermore, a one-day-old girl does not eat Terumah; she drinks milk. The Gemara explains that the concern in Pumbedita was regarding Terumah oil. They wished to anoint the infant with Terumah oil, and anointing is considered equivalent to eating or drinking concerning impurity. Therefore, if the infant was impure, she could not be anointed with Terumah oil. This is derived from the verse, "Do not desecrate the Terumah," which includes anointing and drinking. Thus, the infant in Pumbedita was immersed because her impurity would prevent her from being anointed with Terumah oil.
The Gemara then asks why the Sages only imposed this stringency on Kuti girls. If there is a minority of young girls who see blood, then Jewish girls should also be suspected. Rav explains the difference: "We who derive 'Isha Ve'isha' and are careful if they see blood, do not make a decree for our daughters. They, who do not derive 'Isha Ve'isha' and are not careful if they see blood, we make a decree for their daughters." This means that Jewish people derive from the Torah that even a young girl can become a Nida, and therefore, they are careful to keep their daughters away from sacred items if they see blood. The Kutim, however, do not make this derivation and are lenient. Since they are lenient, the Sages imposed a stricter decree upon them, considering all their daughters impure due to the prevalent minority of young girls who see blood.
The Gemara then asks, "What is this derivation of 'Isha Ve'isha'?" It is taught in a Baraita that the verse concerning Nida states "Ve'isha" (and a woman). This extra "Vav" (and) comes to include even a one-day-old girl who sees blood. However, another verse concerning the impurity of relations (Tum'at Tashmish) also uses "Ve'isha." This verse is understood to apply to a girl of three years and one day, the age at which she can experience Tum'at Tashmish. The Gemara asks: is "Ve'isha" meant to include a one-day-old, or a three-year-old? The resolution is that one of these is a Halacha L'Moshe M'Sinai (a law given to Moses at Sinai) and the verse is merely an Asmachta (a scriptural allusion). Since the verse "Ve'isha" naturally implies "any age," it is more logical to say that the one-day-old rule for Nida is derived from the verse, while the three-year-old rule for Tum'at Tashmish is a Halacha L'Moshe M'Sinai, with the verse serving as an Asmachta.
The Gemara then asks what the extra "Vav" in "Ve'isha" by Tum'at Tashmish comes to teach, if not the three-year-old rule. Rashi explains that this extra "Vav" comes to exclude a man from red secretions. If a woman has a red secretion, she is a Nida. If a man has a red secretion, he is not. This exclusion of a man from red secretions is derived from the "Ve'isha" in the context of Tum'at Tashmish, even though the verse itself is not discussing secretions. The Gemara acknowledges that this is a far-fetched derivation, but explains that when a word is extra and cannot be explained within its immediate context, it is used for a derivation that is as close to the subject as possible.
The Gemara then introduces another challenge: if a one-day-old girl can be a Nida, then she should also be able to become a Zavah (a woman with an irregular uterine discharge). The verse states, "Ve'isha Ki Yazuv Zov Dama Yamim Rabim B'Donat Niddata Temeah Hi" (and a woman, if the flow of her blood is for many days, in the days of her Nida, she is impure). This verse seems to teach that a girl who is at least ten days old can become a Zavah. Why specify ten days? Because Nida lasts seven days, and a Zavah Gedolah (major Zavah) requires three consecutive days of discharge, totaling ten days. The Gemara asks why we need a separate verse for Zavah, as it could be derived from Nida. Once a girl can be a Nida, she should logically also be able to be a Zavah, since Zavah always follows Nidut. The Gemara answers that one might have thought that the strictness of Nida (seven days of impurity for even one drop of blood) would apply to a young girl, but not the leniency of Zavah (one day of impurity for one discharge, or three days for a major Zavah). Therefore, a separate verse is needed to teach that even a young girl can become a Zavah.
The Gemara then reverses the question: why not derive Nida from Zavah? If the Torah tells us a girl can be a Zavah, then she must also be able to be a Nida, as Zavah presupposes Nidut. The Gemara concedes that this is a valid point, meaning the extra "Ve'isha" by Nida is indeed superfluous. Therefore, it is used to exclude a man from red secretions, as previously discussed. This requires two verses to exclude men from red secretions: one for actual red blood (derived from Nida) and one for red seminal discharge (derived from Tum'at Tashmish).
The discussion then shifts to males. How do we know that a young boy, even one day old, can become a Zav or experience seminal impurity? The Baraita states, "Ish Ish" (a man, a man), or according to Rabbi Yishmael son of Rabbi Yochanan ben Beroka, "Lo Zakar" (not a male, but a male). These extra words come to include a male of any age. Similar to the previous discussion, a contradiction arises: another verse teaches that a man becomes impure from seminal discharge at nine years and one day. The Gemara resolves this by stating that the nine-year-old rule is a Halacha L'Moshe M'Sinai, and the verse is an Asmachta. The derivation of "Ish Ish" or "Lo Zakar" to include a one-day-old boy is the actual scriptural derivation. The extra "Vav" in "Ish" in the context of seminal impurity then comes to exclude a woman from white secretions, meaning white discharge only causes impurity for a man, not a woman.
Finally, the Gemara asks why two separate verses are needed, one for males and one for females, to teach about impurity at a young age. Why not derive one from the other? The Gemara explains that males and females have different rules regarding impurity. For example, a male becomes a Zav whether he sees three discharges on three separate days or three discharges on the same day, and he then requires seven clean days. A female, however, only becomes a Zavah if she sees three discharges on three separate days. This difference in stringency means one cannot be derived from the other. Furthermore, a woman can become a Zavah due to "Ones" (involuntary causes, such as jumping), whereas a man does not become a Zav due to Ones; his discharge must be from his body, not an external cause. These fundamental differences necessitate separate scriptural teachings for males and females regarding impurity.