The 1940s brought some new things to zombie films. Unlike other undead, zombies required a human agent to bring them back, an agent who, of course, was evil. During WWII, the ultimate evil was the Adolph Hitler, hence the production of a few nazi zombies movies for propaganda. Borrowing voodoo for their own heinous ends, the Nazis used zombies to build armies, gather intelligence, and plan invasions of the USA. John Carradine played a Nazi scientist building a zombie army in a Louisiana swamp in Revenge of the Zombies (1943).
With the massive industrialization of the US during the 1940s and the decline of mysticism, science and medicine gradually replaced voodoo as the factor that bring about zombies. After WW II, the spectre of atomic warfare was a fear more real than black magic and the undead. Last but not least, rumours of alien visitations (Roswell in 1947), created new fears among the population. Creatures of apocalyptic futures and threatening extra-terrestrials are the creatures of the new genre of science fiction.
In Creature with the Atom Brain (1955), an ex-Nazi scientist is using atomic zombies to aid him in his schemes. In Invisible Invaders (1959), aliens inhabit the bodies of the recently deceased. Zombies now had also taken on a more menacing cast in and of themselves, not just being the slaves of evil masters, as film-makers decided to heighten their scare factor.
Richard Matheson's 1954 post-apocalyptic novel I Am Legend is also considered a pioneer of the modern zombie, despite the creatures being described by the main character as vampires. Employing a stark, almost documentary style, Richard Matheson was one of the first writers to convince us that the undead can lurk in a local supermarket freezer as well as a remote Gothic castle. The story deals with isolation and a worldwide outbreak of a disease causing the population to turn into weak, infected creatures that feed on the blood of the living.
Matherson's novel was adapted into the film The Last Man On Earth in 1964, starring Vincent Price. The book would be adapted twice more The Omega Man (1971) starring Charlton Heston and I Am Legend (2007) starring Will Smith.
Writer-director-producer-editor-actor Edward D. Wood, Jr.'s Plan 9 from Outer Space (1954) may be regarded by many as being one of the worst movie ever made. The "Plan 9" of the title is actually the film's hostile aliens' scenario for invading the Earth via, and we quote here, "long-distance electrodes shot into the pineal and pituitary gland of the recently dead."
Hammer Horror's Plague of the Zombies (1966) is another turning stone in the cinematic zombie, being the first film to show zombies as walking corpses. The film set the standard for zombie make-up to come and green rotting flesh as a standard. However, zombies would not be depicted as being free from their masters until 1967.
In 1968, George Romero took zombie films to the next logical step. What if the zombies didn't have a master? What if they just got up and started attacking people on their own? And if science could make zombies on purpose, why couldn't something happen to make them accidentally?