They feel pain of the forces of decay which eating live brains' neurons cure. Fast and energetic as Trioxin gives them an adrenaline rush. Cooperate with each other to entrap human victims. Alert and respond to stimuli. Retain their intelligence.
Eyes fix on victim.
There's a lot of people dying. It wasn't the dead people rising from graves that outnumbered the living but the people that die everyday of various causes and those that do attack the living leaving fatal bites that either transmit a germ or cause trauma to the victim who become zombies themselves in a matter of hours to days. Maybe the survivors in Day had no idea that there may be survivors elsewhere, perhaps the Northeast, due to failed radio contact. Through human err. They only took over an industrial park area.