[Merlin122]

http://www.imdb.com/title/tt0063350/plotsummary

[quote:d6f3d]Speculation rests on a radiation-covered NASA satellite returning from Venus, but it only remains a speculation. Anyone who dies during the crisis of causes unrelated to brain trauma will return as a flesh-eating zombie, including anyone who has been bitten by a zombie.[/quote:d6f3d]

It turns out he did have an idea for how the zombies were created. Its nice though he didnt mention it in the later films.