Seeing as in order to be Tahor, the Tzara'as needs to spread over the entire body, what does "Eis Kol Or ha'Nega" mean?
Ramban: The Pasuk means that the Tzara'as covers the area that was initially stricken plus the rest of the body from head to foot - from which we can extrapolate that if the initial area that was stricken turns into a Bohak 1 or heals completely, the Nagu'a is Tamei.
A Tahor plague (See Pasuk 39).
Why does the Torah add "le'Chol Mar'eh Einei ha'Kohen"?
Rashi: To preclude a Kohen with impaired eyesight from examining Nega'im.