The story I'm currently working out is a pretty basic idea: zombie apocalypse hits, survivors make their way to safety. I have the details for the most part worked out, including characters, plot points and a hopefully satisfying revelation, but I've encountered a major snag. In my original concept, the "creeps" (the name my characters give the zombies) were created by a biological terrorist attack involving a genetically engineered strain of the rabies virus. It worked for a while, but as my story line evolved, it didn't quite hold water. You see, as the story goes, its revealed that the creeps aren't mindless, slavoring beasts: they operate with a sort of pack mentality. They, as a whole, make up a massive apex predator of sorts, which I feel ups the ante beyond the standard zombie scenario. Problem is, a weaponized strain of rabies can't explain the incredibly powerful pack mentality the creeps exhibit. I know there's that whole "willing suspension of disbelief" thing, but in my opinion I shouldn't rely too heavily on that; after all, if I wouldn't be willing to believe it, why should I demand my reader to? So what I'm asking for is some help trying to come up with a new source for the zombie plague. I'd still like to keep the idea of a terrorist organization or a cult (possibly just some guy with a god complex and too many followers),and I'd like to stay away from mystical fantasy explanations, which have no place in my story. I'm willing to delve into sci-fi, but nothing like aliens or the like. I'm not asking anyone write my story for me; if you post anything, I'm sure it'll get the plot-bunnies bouncing. So, any ideas?