Where the iconic series drew its inspiration from initially, having been a cult following well before it was ever considered for the screen. While the first season of the show follows the source material closely there was a disagreement between the producers and the creators as far as which direction the series was going to take. Had read some time ago, In order for Robert Kirkman to reach a deal with the production company it was stipulated that there would have to be a cause revealed eventually as to why there were zombies. Kirkman being a true Romero enthusiast had no intentions of designating any specific cause as to keep the mystery alive. So inorder to reach a deal supposedly he threw out the suggestion that it would be revealed that aliens were somehow the cause, it appears they had expected there to be a reveal or at least a nod by the close of the first season, being they had no intention of going this route with it there was contention.