Gemara

(Mishnah): Red is like blood of a wound.

(Ami Vardina'ah): It is like the blood of the pinky if it was bruised, healed and bruised again, of a bachelor less than 20.

Question (Mishnah): She is Toleh on her son or husband.

Her husband is not a bachelor, yet she can be Toleh on him! (His blood resembles Dam Nidah.)

Answer (Rav Nachman bar Yitzchak): The case is, he performed Chupah, but did not yet have Bi'ah with her.

58b (Mishnah): She may be Toleh on anything possible, e.g.:

If she slaughtered a Chayah, Behemah or bird;

If she was engaged in Kesamim or sat near people engaging in them.

If she killed a louse, she may be Toleh on it.

(R. Chanina ben Antignos): She may be Toleh up to a Gris (bean), and even if she did not kill a louse.

She may be Toleh on her son or husband.

If she has a wound (even if it has a scab on it) that can open up and emit blood, she is Toleh on it.

R. Akiva: Chachamim enacted to be Metamei women due to Kesamim, but to be lenient (whenever we can be Toleh);

"V'Ishah Ki Sihyeh Zavah Dam Yihyeh Zovah bi'Vesarah" -- (mid'Oraisa she is Tamei due to) blood, but not a Kesem.

(Beraisa): If she passed through a meat market and is unsure whether or not blood splashed on her, she may be Toleh on it;

If she is unsure whether or not she passed through a meat market, she is Teme'ah.

Rishonim

Rambam (Hilchos Isurei Bi'ah 9:22): If she had a wound, even if it healed, if it could open and emit blood, if blood is found on her skin she is Toleh on the wound. The same applies to all similar cases.

Rambam (24): She may be Toleh on her son or husband, if they engaged in blood or their hands were dirty, or they had wounds. She is Toleh on them and assumes that they touched her without her knowledge, and the blood is due to them.

Magid Mishneh: The Ramban says that if her son or husband engaged in blood but no blood was found on them, she is not Toleh on them.

Beis Yosef (YD 190 DH v'Chasav ha'Ramban): However, if they engaged in something that splashes, such as Shechitah, surely even if no blood was found on them, she is Toleh on them.

Question: Hagahos Maimoniyos brings from Sefer ha'Terumah and the Mordechai that the first three of the seven clean days must be totally clean. If she found a Kesem, even though she could be Toleh on a wound, she may not. If she finds less than a Gris, may she be Toleh on a louse?

Answer (Terumas ha'Deshen 249): It seems that they say that she may not be Toleh only on a wound or a bird, i.e. if she finds more than a Gris. If she finds less, she need not be Toleh on a wound. She may be Toleh on a louse. If so, it is reasonable that a large Kesem, which not so common, and she can be careful about it, we are not Toleh on it in the first three days. Surely, always we may be Toleh a Kesem less than a Gris. If not, no woman would be able to become Tahor. R. Shimon ben Gamliel (58b) said 'according to me, a woman cannot be permitted to her husband. Beds always have many drops of blood from lice!' I.e. he allows to be Toleh on a louse, even less than a Gris, only if she killed one. This shows that one cannot be careful to avoid (blood of) lice. Likewise, if we cannot be Toleh on a louse in the first three days, no woman can count seven clean days.

Poskim

Shulchan Aruch (YD 190:18): Since Kesamim are mid'Rabanan, we are lenient about them and attribute to anything possible. If she slaughtered an animal, or bird, or engaged in Kesamim, or sat next to people engaged in them, or passed through a butchers' market and found blood on her garments, she is Toleh on them, and she is Tehorah. However, if she found blood on her skin, she is Toleh only if she has a wound in a place from which the blood could drip to there. Even if the wound healed, if it could open and emit blood through scratching, she is Toleh on it, even if now there is a scab over it and it does not drip blood.

Beis Yosef (DH u'Mah she'Chasav v'Afilu): The Rashba says that even if now there is a scab over it and it does not drip blood, she is Toleh on it. Perhaps the wound opened without her knowledge. Also Mordechai, Semag and Sefer ha'Terumah say so.

Beis Yosef (DH Kasav): The Mordechai says that if a woman used to see blood due to Bi'ah, she is believed to say that she has a wound in that area from which blood comes. She is Toleh on it only if she knows that blood comes from there. She may not be Toleh from Safek. However, at the time of the Veses, even if she knows that blood is coming from the wound, she may not be Toleh on it. However, Kesamim that she finds on her garments are Tehorim. She is Toleh them on a wound, even if she is unsure whether it emits blood. Chachamim are lenient about Kesamim because they are mid'Rabanan. This is from Sefer ha'Terumah, who concludes 'a Stam wound emits blood sometimes. If she counts seven clean days, reasoning teaches that from three days and onwards, her Kesamim are Tehorim and she may be Toleh them on a wound, and she does not lose days that she counted. However, she must know that the blood from the Makor ceased. This is why she needs three totally clean days. The Terumas ha'Deshen allows to be Toleh on a louse even in the first three days.

Beis Yosef (DH v'Chasav): The Rashba says that if she engaged in Kesamim with her hands, she is Toleh on them even for Kesamim on her skin. Even though she cannot be Toleh Kesamim on her skin even if she walked through a butchers' market, if she engaged in Kesamim with her hands, she is Toleh on them blood she finds on her hands. Similarly, if she has a wound, she is Toleh on it even if blood is found only on her skin.

Pischei Teshuvah (31): Chamudei Daniel says that if a woman's nose often bleeds due to a small hit, she may be Toleh on it. Sidrei Taharah (52) says that nowadays many women smell snuff, and when they blow the mucus from their nose into a linen handkerchief, there is a red stain, they may be Toleh on this. Also, sometimes they wrap infants in red garments, and due to urine some of the red dye comes onto her garment or sheet. Surely she is Toleh on this. Therefore, if she wears red socks, and when she sweats her garment gets dirty from them, she is Toleh on this. Everything is according to the Rav's judgment. Chamudei Daniel says that if she slaughtered, we do not require that the blood or Kesamim could get to there. Rather, if her hand became dirty (with blood), we are Toleh that it came to the garment from her hands, for hands go places. Therefore, butchers' wives can be Toleh all their Kesherim to blood that dirtied their hands.

Shulchan Aruch (19): Just like she is Toleh on it (a wound), she is Toleh on her son or husband, if they engaged in Kesamim or they have a wound, for it is normal for them to touch her.

Pischei Teshuvah (32): Sidrei Taharah says that even if the blood is found only on her skin, she is Toleh on her son or husband. In the Seifa, when blood was not found on her son or husband, just they engaged in something that splashes, she is Toleh only if the blood was found also on her garment. It is no better than when she passed through a butchers' market. Also Chachmas Adam says so.

The questioner (in Chasam Sofer YD 186): The Shulchan Aruch connotes that she is Toleh on her son or husband just like on a wound on her body. Even if (there is a scab, just) it could become exposed, she is Toleh on them. What is the source for this? The Mishnah discusses being Toleh on her son or husband, and later says 'if she has a wound that could become exposed.' This connotes only if it is on her, but she is Toleh on her son or husband only when they have a wound that emits blood. Also the Rambam connotes like this.

Chasam Sofer: Also the Ra'avad (in Ba'alei ha'Nefesh) and Ramban connote like this. However, this is difficult, for then in the Mishnah 'she is Toleh on her son or husband' does not apply to what follows this "if she had a wound', rather, on what precedes it 'just like she is Toleh if she engaged in blood, Kesamim or lice, so she is Toleh on her son or husband.' The Reisha did not mention a wound at all! Even though a Kal va'Chomer (from other things she is Toleh on) teaches that she is Toleh on an open wound on her body that emits blood, a wound was not mentioned regarding her son or husband. If so, what was the question (19b) 'how is she is Toleh on her husband', and why did we need to give a difficult answer? The Mishnah did not say that she is Toleh on her husband's wound. Perhaps she is Toleh on him just like (i.e. whenever she could be) on herself, and indeed she is not Toleh on his wound (because the blood looks different)! Rather, it was clear to the Gemara that 'she is Toleh on her son or husband' applies to what follows 'if she had a wound, and it can open...' Perhaps 'she is Toleh on her son or husband' begins a new Mishnah, just in our Gemaros it was printed with the Reisha. I prefer to say that the text said 'if they had a wound...' The Beis Yosef brought this law from the Rashba. Also Semag, Sefer ha'Terumah, the Mordechai and R. Yerucham say so. I lean to say that the Rambam, Ra'avad and Ramban, which are not explicit, can agree with these Poskim who are explicitly lenient about Kesamim, which are mid'Rabanan. Also, one cannot refute my proof from the Gemara.

Shulchan Aruch (ibid.): If they engaged in Kesamim but no blood was found on them, she may be Toleh on them only if they engaged in something that normally splashes, e.g. Shechitah or similar matters.

Shach (33, from the Bach): It seems that this is if we do not know that blood was found on them after they engaged, but if we know that there was blood on them, even though now there is not, we are Toleh that when she touched them, there was blood on them.

Rema: The same applies to if she lied in a bed in which were women with wounds on their body. She may be Toleh on them like on her son and husband.

Shach (34, and Drishah): The Gemara discusses her son and husband, for it is normal for them to touch her.