(יח) וְאִשָּׁ֕ה אֲשֶׁ֨ר יִשְׁכַּ֥ב אִ֛ישׁ אֹתָ֖הּ שִׁכְבַת־זָ֑רַע וְרָחֲצ֣וּ בַמַּ֔יִם וְטָמְא֖וּ עַד־הָעָֽרֶב׃
(18) And if a man has carnal relations with a woman, they shall bathe in water and remain unclean until evening.
"ורחצו במים"
RASHI
"They shall both lave themselves in water"
It is a decree of the King that a woman should become unclean through sexual intercourse. The reason is not because of the law of “one who touches שכבת זרע” (who, according to Leviticus 22:7 is unclean; cf. Sifra on that verse) for in her case it is מגע בית הסתרים and this does not render her unclean (Niddah 41b).English
Mishnah Niddah 5:1
For a Caesarean section, one does not sit for the days of impurity or the days of purity, and one is not liable to bring a sacrifice on its account. Rabbi Shimon says: it is like one who was birthed [regularly]. All women render impurity [from when the blood is] in the outer chamber, as the verse says (Leviticus 15), "Blood will be her flow within her flesh." But a zav [a male who has certain types of atypical genital discharges, which render him impure] and a ba'al keri [a male who has had a seminal emission but has yet to purify himself by immersion in a mikveh] do not render impurity until their impurity emerges to the outside.
Mishnah Niddah 8:3
It happened that a woman came before Rabbi Akiva. She said to him, "I saw a [blood] stain. He said to her, "Perhaps you had a wound?" She said to him, "Yes, but it has healed." He said to her, "Perhaps it [was a wound that] could open up and produce blood." She said to him, "Yes." And Rabbi Akiva declared her pure. He saw his students looking at one other [in astonishment]. He said to them, "Why is this matter difficult in your eyes? For the Sages did not state this matter to be stringent, but rather to be lenient, for it is said, 'And if a woman has a flow, blood will be her flow within her flesh,' (Leviticus 15:19); blood [renders a woman impure, according to the Torah], but not a [blood] stain."
Sifra, Metzora Parashat Zavim, Section 4 11
11) (Vayikra 15:20) "And all that she lies upon in her niddah state shall be unclean, and all that she sits upon shall be unclean": I might think that she confers tumah upon a mishkav that is not distinctive for reclining upon and upon a moshav (a seat) that is not distinctive for sitting upon. It is, therefore, written (Vayikra 15:21) "And whoever touches her (distinctive) mishkav shall wash his clothes."
Berakhot 4a:9
And the other Sage said: David said the following before the Holy One, Blessed be He: Master of the Universe, am I not pious? For all of the kings of the East and the West sit in groups befitting their honored status, but I sit as a judge who issues rulings for the people. Women come with questions of ritual impurity and my hands become soiled withtheir blood as I labor to determine whether or not it is blood of impurity and she has menstruating woman status, and with a fetus that miscarried at a stage of development before it was clear whether or not it is considered a birth, and with placenta, which women sometimes discharge unrelated to the birth of a child (see Leviticus 15:19–30 with regard to blood, and 12:1–8 with regard to miscarriage and placenta). King David went to all this trouble in order to render a woman ritually pure and consequently permitted to her husband. If, after examination, a Sage declares the woman ritually pure, she is permitted to be with her husband, which leads to increased love and affection, and ultimately to procreation (Rabbi Yoshiyahu Pinto). And not only do I engage in activity considered to be beneath the station of a king, but I consult my teacher, Mefivoshet, son of King Saul’s son, Jonathan, with regard to everything that I do. I say to him: Mefivoshet, my teacher, did I decide properly? Did I convict properly? Did I acquit properly? Did I rule ritually pure properly? Did I rule ritually impure properly? And I was not embarrassed.Forgoing royal dignity should make me worthy to be called pious.
IBN EZRA
you will keep disassociated Some people explain that this word is missing a heh , and that it means “you will warn them”. This, however, is unlikely, because a heh does not indicate diphthongs — only at the end of a word does it become silent or disappear. Rather, the word is related to “they will abstain from the sacrifices…” [22:2] and means, “you will distance them”. The word “Nazirite” — one who distances himself from worldly desires — stems from the same root. This verb’s form parallels that of “You shall divide [actually, “When you divide” — Translator] the land by lot” [Ezekiel 45:1]: the root nun disappears in the dagesh .