Invisible Invaders -- Edward L. Cahn
George Romero rightfully gets credit for being the father of the modern zombie movie (as opposed to Voodoo zombies). I can't help but think that Invisible Invaders held some inspiration for him. Romero was 19 at the time of its release, a perfect age to notice a good concept in an imperfect movie and log it away for later use.
Invisible Invaders is about an invisible alien force who plan on taking over the world. Initially, one of these aliens inhabits a recently deceased corpse (played by the always awesome John Carradine) to communicate to one prominent scientist that he must spread the word of the impending invasion or face annihilation. This makes some sense as the alien as some visual aids and, being invisible, it would be hard to get the scientist's attention. But for whatever reason, the invading force keeps taking up residence in dead individuals and wandering the earth looking for humans (even more perplexing is why they stumble around mindlessly when the John Carradine alien was not only eloquent, but well-coordinated). If an invading force is going to attack, why give up your greatest asset? It allows our heroes to capture one and eventually figure out how to kill them and shows them where they are.
Invisible Invaders is a pretty poor entry in the nuclear horror genre, but it holds several elements for what would become Night of the Living Dead (a quick google search of "George Romero" and "Invisible Invaders" shows I'm not the only one who thinks this), so I won't knock it too hard.