Mishnah Niddah 7:1: The blood of a niddah [a female who has menstrual discharges which render her impure] and the flesh of a corpse render impurity when damp, and render impurity when dry. But the discharge [of a zav, a male who has certain types of atypical genital discharges which render him impure], and the phlegm [of a zav], and the spit [of a zav], and a [dead] vermin, and carrion, and semen render impurity when damp but do not render impurity when dry. If they can be steeped [in water] and thereby revert to their original condition, they render impurity when wet and render impurity when dry. And how long must they be steeped? In lukewarm water, from a time to that time [twenty four hours later]. Rabbi Yose says: the dry flesh of a corpse, if it cannot be steeped and thereby reverted to its original condition, is pure.

דַּם הַנִּדָּה וּבְשַׂר הַמֵּת, מְטַמְּאִין לַחִין וּמְטַמְּאִין יְבֵשִׁין. אֲבָל הַזּוֹב וְהַנִּיעַ וְהָרֹק וְהַשֶּׁרֶץ וְהַנְּבֵלָה וְהַשִּׁכְבַת זֶרַע, מְטַמְּאִין לַחִין וְאֵין מְטַמְּאִין יְבֵשִׁין. וְאִם יְכוֹלִין לְהִשָּׁרוֹת וְלַחֲזֹר לִכְמוֹת שֶׁהֵן, מְטַמְּאִין לַחִין וּמְטַמְּאִין יְבֵשִׁין. וְכַמָּה הִיא שְׁרִיָּתָן. בְּפוֹשְׁרִין, מֵעֵת לְעֵת. רַבִּי יוֹסֵי אוֹמֵר, בְּשַׂר הַמֵּת יָבֵשׁ וְאֵינוֹ יָכוֹל לְהִשָּׁרוֹת וְלַחֲזֹר לִכְמוֹת שֶׁהָיָה, טָהוֹר:

Mishnah Niddah 7:2: A [dead] vermin found in an alleyway retroactively renders impure [anything that was in the alleyway, until the time it was found and] since such a time regarding which one says, "I examined this alleyway and there was no vermin in it," or since the hour at which it was [last] cleaned. Similarly regarding a [blood] stain found on a [woman's] robe, it retroactively renders impure [any pure items which the woman handled, until the time the stain was found and] since such a time regarding which one says, "I examined this robe and there was no stain on it," or since the hour it was [last] washed. And it [i.e. the vermin or the stain, retroactively] renders impurity, whether it is damp or dry. Rabbi Shimon says: if it is dry it renders impurity retroactively, but if it is wet it renders impurity only since the hour at which one could go back and it would be damp [i.e. regarding the vermin, the hour at which, if it had fallen then or later, it could still be damp currently].

הַשֶּׁרֶץ שֶׁנִּמְצָא בַּמָּבוֹי, מְטַמֵּא לְמַפְרֵעַ, עַד שֶׁיֹּאמַר בָּדַקְתִּי אֶת הַמָּבוֹי הַזֶּה וְלֹא הָיָה בוֹ שֶׁרֶץ, אוֹ עַד שְׁעַת כִּבּוּד. וְכֵן כֶּתֶם שֶׁנִּמְצָא בֶחָלוּק, מְטַמֵּא לְמַפְרֵעַ, עַד שֶׁיֹּאמַר בָּדַקְתִּי אֶת הֶחָלוּק הַזֶּה וְלֹא הָיָה בוֹ כֶתֶם, אוֹ עַד שְׁעַת הַכִּבּוּס. וּמְטַמֵּא בֵּין לַח בֵּין יָבֵשׁ. רַבִּי שִׁמְעוֹן אוֹמֵר, הַיָּבֵשׁ מְטַמֵּא לְמַפְרֵעַ, וְהַלַּח אֵינוֹ מְטַמֵּא אֶלָּא עַד שָׁעָה שֶׁהוּא יָכוֹל לַחֲזוֹר וְלִהְיוֹת לַח: