When must the Nega return in order to render the house Tamei?
Rashi: We learn from a Gezeirah Shavah "Shav" "Yashuv" (from Pasuk 37) that it is only after seven days (after having replaced the afflicted stones) that the returning Nega renders the house Tamei 1
Riva: Rashi holds that there is no difference between a Nega that returns and one that spreads. After the first or second week of Hesger, it obligates replacing the stones and the mortar, and another week of Hesger. After the third week, it obligates destroying the house. The house becomes Tahor only if the Nega does not return.
What is the meaning of "u'Parach"?
Ramban (citing The Sifra): It means that the plague erupts. 1 It is a new plague that erupts on different stones and on different mortar and is nevertheless Tamei just like the initial plague, 2 thereby adding to the incredible phenomenon of Nig'ei Batim. 3 It is a Ru'ach Ra that will not leave; it will always scare the inhabitants.
Why does the Pasuk add the word "Pasah ha'Nega ba'Bayis"?