When out exploring the world, sometimes the sky turns red and a strange sound fills the air. During these times, “Events,” all zombies are fast and more alert.
However, there is a special type of zombie called a tweaker. Tweakers are ALWAYS fast. Super fast. When idle, tweakers simply stand still and make a characteristic gurgling noise. If you shine your light by them or make a noise (gun shot, weapon attack, or footsteps) too close to them, they will be alerted to your presence and come chasing after you.