These are not the living dead, at least not in the traditional sense, but living humans driven into unthinking frenzy and stripped of their personality by a virus. Even a scratch or the slightest contact with infected blood is enough to transmit the virulent disease, which has an incubation time measured in mere hours. It spreads quickly, overwhelming the civil authorities before they can put any sort of effective quarantine in place.
And these are not the shambling, dimly aware zombies of midnight movies. The Infected are fast, cunning predators. The virus pushes their bodies to their limits and beyond, imbuing them with adrenaline-fueled speed and heightening all their senses. There is nowhere to hide, and nowhere to run. The Infected don’t get tired, they don’t care if their feet are worn raw and bloody, and they don’t give up.
—"Pandemic - Apocalypse," The End of the World: Zombie Apocalypse
"I can't get over how fast they all are! It's not even fair, I'm calling zombie bullshit on that, you know? They're not... allowed to be so fast!"
— Zoey, Left 4 Dead
"What do you think the odds are that the swamp people are alive?
"Alive and infected, or alive and not zombies?"
— Nick and Rochelle, Left 4 Dead 2
As you're darting from place to place in the Alcove like some sort of grave-robbing cheetah, you encounter something moving even faster than you.
"It's a zmobie, but it's not your grandfather's zmobie (George Romero is your grandfather, right?) — It's one of those new-fangled super fast zmobies. And man, does it have a mean look on its face."
— Flavor text for the modern zmobie (and yes, the mispelling is intentional), Kingdom of Loathing
