Mishnah Niddah 8:2: A woman [who found a stain] may attribute it to any cause to which she is able to attribute it: [for instance] if she had slaughtered a beast, or a wild animal or a fowl, or if she was handling [blood] stains, or if she sat beside those who were handling them. If she killed a louse she may attribute [the bloodstain] to it. Up to how large a stain may be attributed [to a louse]? Rabbi Chanina ben Antignos says: up to the size of a split bean, and even if she did not kill it. And she may attribute it to her son or to her husband [i.e. if one of them has a wound]. If she has a wound that could open and produce blood she may attribute [the bloodstain] to it.

וְתוֹלָה בְכָל דָּבָר שֶׁהִיא יְכוֹלָה לִתְלוֹת. שָׁחֲטָה בְהֵמָה, חַיָּה וָעוֹף, נִתְעַסְּקָה בִכְתָמִים אוֹ שֶׁיָּשְׁבָה בְצַד הָעֲסוּקִים בָּהֶן, הָרְגָה מַאֲכֹלֶת, הֲרֵי זוֹ תּוֹלָה בָהּ. עַד כַּמָּה הִיא תוֹלָה. רַבִּי חֲנִינָא בֶן אַנְטִיגְנוֹס אוֹמֵר, עַד כַּגְּרִיס שֶׁל פּוֹל, וְאַף עַל פִּי שֶׁלֹּא הָרְגָה. וְתוֹלָה בִבְנָהּ אוֹ בְּבַעְלָהּ. אִם יֶשׁ בָּהּ מַכָּה וְהִיא יְכוֹלָה לְהִגָּלַע וּלְהוֹצִיא דָם, הֲרֵי זוֹ תּוֹלָה בָהּ:

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."

מַעֲשֶׂה בְאִשָּׁה אַחַת שֶׁבָּאת לִפְנֵי רַבִּי עֲקִיבָא, אָמְרָה לוֹ, רָאִיתִי כָתֶם. אָמַר לָהּ, שֶׁמָּא מַכָּה הָיְתָה בִיךְ. אָמְרָה לוֹ, הֵן, וְחָיְתָה. אָמַר לָהּ, שֶׁמָּא יְכוֹלָה לְהִגָּלַע וּלְהוֹצִיא דָם. אָמְרָה לוֹ, הֵן. וְטִהֲרָהּ רַבִּי עֲקִיבָא. רָאָה תַלְמִידָיו מִסְתַּכְּלִין זֶה בָזֶה. אָמַר לָהֶם, מַה הַדָּבָר קָשֶׁה בְעֵינֵיכֶם. שֶׁלֹּא אָמְרוּ חֲכָמִים הַדָּבָר לְהַחְמִיר אֶלָּא לְהָקֵל, שֶׁנֶּאֱמַר (ויקרא טו), וְאִשָּׁה כִּי תִהְיֶה זָבָה דָּם יִהְיֶה זֹבָהּ בִּבְשָׂרָהּ, דָּם וְלֹא כָתֶם: