The ravenous antagonists in 28 days Later were not zombies, they're "crazies". They are infected with the synthetic pseudo-virus "Rage", developed by scientists who infected chimpanzees in an effort to find a cure for rage and anger in human beings. "To be able to cure, we must first understand". At least, that's what I gathered. They were not undead, they are susceptible to everything lethal to normal humans, only they are highly infectious and oblivious to pain. Their minds may still be intact, but they are blinding by the most pure and focused rage imaginable, yet don't seem to go for fellow crazies.
The Dawn of the Dead zombies were a new twist, begging the question "If their bodies are still intact, why shouldn't they be able to use them to their full extent?" The virus, like so many other, real infectious pathogens, change the behavior of their host to facilitate the continues spread of the disease.
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