It turns out that the aliens that abducted Marion were actually trying to stop the zombies from spreading, and that the rain wasn't acid, but that it was healing the zombies. Then they take off in the wall, which was actually a spaceship, and leave, having stopped the spread. BUT Wayne manages to fly over the alien wall in his plane and he is infected, because it was airborne. He becomes a zombie, spreads the disease and Rene and a handful of others survive. You then see that, at the end, Rene has all of the zombies in a pen outside her farm and is waiting for the aliens to come back.