Reference for Kelim 1:1
אֲבוֹת הַטֻּמְאוֹת, הַשֶּׁרֶץ, וְשִׁכְבַת זֶרַע, וּטְמֵא מֵת, וְהַמְּצֹרָע בִּימֵי סָפְרוֹ, וּמֵי חַטָּאת שֶׁאֵין בָּהֶם כְּדֵי הַזָּיָה, הֲרֵי אֵלּוּ מְטַמְּאִין אָדָם וְכֵלִים בְּמַגָּע, וּכְלֵי חֶרֶשׂ בַּאֲוִיר, וְאֵינָם מְטַמְּאִין בְּמַשָּׂא:
The Origins of impurity: the <em>sheretz</em> [one of eight creeping creatures, whose identities are debated, and whose carcasses transmit ritual impurity upon contact], semen, that which is rendered impure by a dead body, the <em>metzora</em> [victim of a Divinely inflicted skin disease] while he is in his days of counting, a small amount of purifying water that is not enough to perform the sprinkling ceremony - can render persons and vessels impure through touch, and pottery vessels through the air, but they do not render impure when they are carried.