The Walking Dead; 1.03 "Tell It to the Frogs" (open spoilers)

[emarkp]

@Tengu

So these have to be proper Romero zombies - being dead with an intact brain is enough to make you a zombie.

This is fully addressed in the comic, and actually has an important plot point (I’ll address this question but not the plot point)

Indeed, these are Romero zombies. Everyone dead with a sufficiently intact brain rises.