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 dont get tired, they dont care if their feet are worn raw and bloody, and they dont give up.
—"Pandemic - Apocalypse," The End of the World: Zombie Apocalypse