Mishnah for Negaim 4:3
יֵשׁ בְּשֵׂעָר לָבָן מַה שֶּׁאֵין בְּמִחְיָה, וְיֵשׁ בְּמִחְיָה מַה שֶּׁאֵין בְּשֵׂעָר לָבָן. שֶׁשֵּׂעָר לָבָן מְטַמֵּא בַשְּׁחִין וּבַמִּכְוָה, בִּמְכֻנָּס וּבִמְפֻזָּר, בִּמְבֻצָּר וְשֶׁלֹּא בִמְבֻצָּר. יֵשׁ בְּמִחְיָה, שֶׁהַמִּחְיָה מְטַמָּא בַקָּרַחַת וּבַגַּבַּחַת, הֲפוּכָה וְשֶׁלֹּא הֲפוּכָה, וּמְעַכֶּבֶת אֶת הַהוֹפֵךְ כֻּלּוֹ לָבָן, וּמְטַמָּא בְכָל מַרְאֶה, מַה שֶּׁאֵין כֵּן בְּשֵׂעָר לָבָן:
There are [laws] by white hair that a healthy patch of skin does not have, and there are [laws] by a healthy patch of skin that white hair does not have. For white hair makes [the <i>Nega</i>] impure if it was in [the area of] a boil or burn, [if the hair was] concentrated or scattered, [if it was] surrounded [by the <i>Nega</i>] or not surrounded. There are [also unique laws] by a healthy patch of skin, for a healthy patch of skin makes a <i>Karachat</i> [<i>Nega</i> that balds the back of the head] or <i>Gabachat</i> [<i>Nega</i> that balds the top of the head], whether [the patch] was preexisting or not preexisting, and it prevents [purification of] someone who has turned completely white [by a <i>Nega</i>] and makes impure with any appearance, which is not true by white hair.