Well again those questions would all be better answered if we knew which cause created the initiation of that specific Z-Day, be it Politically created: in which case it could be becuase of a need for more power, to create a balance and show who is in charge......which we all know would fail becuase the USDZA would have to intervene, save the horrible politics from their "fail" and take control democratically.
or Science gone wrong: in which case most time would be spent protecting people and working on a cure after locating the cause.
and of course GOD: god would have to exist in order for that to be a cause (thus the lower casing of his name in this description) and even if he is, not saying im set on the fact that he isnt, but if he is.......why zombies? why not water again? would be kinda backtracking if he were to destroy his people using partially destroyed people? that would be more devil like work (if you believe in that kinda thing)