In the classic horror movie 28 days later, zombies are
referred to as the infected. The get
infected when they come in contact with the blood of an infected person or
animal or get bit by and infected person or animal. The plague starts when some infected monkeys
in a lab are released.
Originally a zombie referred to a person who was unaware of his or her surroundings. The earliest example of these originated where witch doctors poisoned people into a deathly state similar to a coma. They would then snatch them from the graves and use them as slaves, constantly drugging them to keep them 'zombified'. These slaves can be made to work on farmland or be used to kill others.
Then an explosion of cannibalistic, mindless, walking corpses occurred in film. These were often called the living dead, or the undead. They soon adopted the name zombies in similarity to the witch doctors who made the 'dead' rise in earlier tales. They were not the first coined term for these new breed of monsters.
Today the term has expanded to include the 'running' zombie. The difference between 28 days later infected to WWZ is minimal at best. Much like the voodoo zombie you don't have to be dead to be classified as a zombie.
28 days later is a great zombie movie, but the infected are not zombies. While the movie fits fairly neatly into the genre (aside from the running aspect, which mainly heightens the horror), the antagonists are not actually zombies. Zombies are categorized as part of the undead family, and the infected are not, nor have they ever been, dead. That’s like saying you can have living ghosts (which I guess would be a psychic projection or something and therefore wouldn’t be a ghost). Another example would be the things in The Cell (the book). They hear a certain tone in their ear and become inhuman, pack-forming entities, that attempt to spread their condition. That does not make them zombies. Conversely, the entities in The Strain are undead, and the lower forms at least are zombie like in behavior, even if the behavior is caused by parasites and not “normal” zombie methods.
If can be killed without headshot its not a zombie. You cant kill zombie by cuting off his hand end letting it bleda to death. They are, as their name suggests, people that are infected with the Rage Virus, causing mindless, uncontrollable rage and aggression.
Frank, after a drop of infected blood fell in his eye, took a few moments before the Virus began to take over, while Clifton became Infected almost instantly after an Infected vomited blood directly into his face.
So if they didnt die and come back to life they are not Zombies, just infected.