The description of a "zombie" is basically just a mindless being, and I believe our world is already full of them. However if you're looking at the traditional zombie definition, meaning a flesh/brain eating diseased and undead human, then it's still possible. Brain diseases are discovered everyday who's to say that sometime that a disease closely resembling the zombie disease won't come about. Also, with how much medicines and drugs are being created, I'm sure zombies will eventually be produced out of something like that. After all that was how zombies came about in I Am Legend and there was that dude in Florida about a year ago with the bath salts