Zombies are scientifically impossible. A rotting corpse, if it could get up and walk would have alot of trouble due to deterriorating muscle, lack of blood flow (would freeze in winter) and functioning nervous system, brain etc probably none. BUT, 28 weeks later esque "zombies", pretty possible i'd say. Alot of parasites out there that can take over the entire brain. Aka uroboros from resident evil 5, although fictional it's a half decent example. Aliens, yeh totally. There is no way we are the only sentient life in a universe we have no clue of its size. Hell, we haven't even explored any of the ocean pretty much.