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 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 Toras Kohanim): 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 "ba'Bayis"?
Ramban (citing Toras Kohanim): To teach us that it is Tamei even if it is in a different part of the room and even if the plague takes on an appearance that is different than the first one.