Blood from the מקור, felt (הרגשה) at the moment of עקירה, renders טמאה — even a drop כחרדל — and she counts seven clean days (ז׳ נקיים) by the חומרא דרבי זירא. The foundation of all טהרת המשפחה (Shulchan Aruch, Yoreh De'ah 183 — 1 se'if)
הָאִשָּׁה שֶׁיָּצָא דָּם מִמְּקוֹרָהּ, בֵּין בְּאֹנֶס בֵּין בְּרָצוֹן, טְמֵאָה — וְהוּא שֶׁתַּרְגִּישׁ בּוֹ בְּצֵאתוֹ; וַאֲפִלּוּ לֹא יָצָא לַחוּץ אֶלָּא שֶׁנֶּעֱקַר מִמְּקוֹמוֹ, אֲפִלּוּ טִפָּה כְּחַרְדָּל — צְרִיכָה לֵישֵׁב שִׁבְעָה נְקִיִּים. הַגָּה: וְאֵין חִלּוּק בְּאִסּוּר נִדָּה בֵּין פְּנוּיָה לִנְשׂוּאָה.
A woman whose blood left her מקור (her uterine source), whether באונס (by force) or ברצון (naturally), is טמאה — provided that she feels it (הרגשה) as it departs; and even if it did not exit outward, once it has detached from its place (נעקר), even a drop כחרדל, she counts seven clean days (ז׳ נקיים). Rama: there is no distinction, in the prohibition of nidda, between a single and a married woman.
Shulchan Aruch, Yoreh De'ah 183:1LEVEL 01
Hebrew text of the single se'if with a fluent English translation. מקור, הרגשה, עקירה, the drop כחרדל and the foundation of the seven clean days (ז׳ נקיים) explained with dignity and practical cases of טהרת המשפחה.
📖 StudyLEVEL 02
In-depth pilpul: נדה דאורייתא vs זבה and the חומרא דרבי זירא (Niddah ס״ו.), the three kinds of הרגשה (PT sk1), the עקירה / יציאה debate (PT sk2, Brit Avraham), the derashah מקור דמיה (Shach), and the universal scope of the prohibition (Rivash, karet).
📖 StudyLEVEL 03
Comparative tables (הרגשה דאורייתא / כתם דרבנן, נדה / זבה, עקירה / יציאה), golden rules, cross-references (sim. ק״צ, קפ״ח, קצ״ו) and memorization of the foundation of the seven clean days.
📖 StudyLEVEL 04
Two parts: the shitah of the Tzemach Tzedek and Chabad (Daat HaRav), then the halacha lema'asse of the other poskim — Beit Yosef, Rama, Shach, Taz, Sidrei Tahara, Chochmat Adam, Aruch haShulchan, Taharat haBayit (Rav Ovadia), Shiurei Shevet haLevi, Badei haShulchan.
📖 StudyOriginally, the Torah's נדה required seven days from the sighting of blood, then immersion, whereas the זבה (a woman who saw blood outside her nidda days) required seven clean days. The Gemara (Niddah 66a) records the חומרא דרבי זירא: בנות ישראל החמירו על עצמן — the daughters of Israel accepted to treat any sighting of blood, even a drop, even during nidda days, as a possible ספק זיבה. Therefore, in practice, one always counts seven clean days before immersion (the details of the count in sim. קצ״ו). For your own situation, consult your Rav.
The הרגשה — the sensation that the blood has detached from its place and is leaving — is the דאורייתא condition for impurity. The Pitchei Teshuva (sk1) distinguishes three kinds: שנזדעזע גופה (the body shudders), שנפתח מקורה (the opening of the makor), and the sensation of a דבר לח זב (a moist flow). A כתם — a bloodstain found without any הרגשה — is by contrast a דרבנן institution (its own laws, sim. ק״צ). This distinction is one of the foundations of the whole sugya. For your own situation, consult your Rav.
Yes: even a single drop like a mustard seed (כחרדל) renders טמאה (Pitchei Teshuva sk3). But only blood from the מקור (the uterine source) carries this status — the Shach derives it from the verse וְהִיא גִלְּתָה אֶת מְקוֹר דָּמֶיהָ. Bleeding from another origin (a wound, מכה) is a separate matter, to be examined case by case. The prohibition of nidda is moreover universal: there is no distinction between a single and a married woman (Rivash, Rama), for כל הבא על הנדה חייב כרת. For your own situation, consult your Rav.