If there are space aliens out there, they're probably sitting around filing their talons or farting cyanide gas or something.

They don't have to do anything to us, we're well on our way to killing ourselves off. Then, all they have to do is vaporize what's left of the dead bodies (the count will depend on how fast we die off), and move in.

Of course, the alternative is if the zombies kill us off first, then the aliens will have to deal with them. But it shouldn't be too much trouble, I've heard they don't move very fast.

Sue