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.