1. They awkwardly drag themselves towards things they want to put in their mouths without any possibility of distraction.
2. They bite.
3. Despite a lack of motor skills or ability to use tools, their glassy eyed determination gets them through complex locks, doors, bars and walls.
4. Being covered in body fluids is no big thing to them (ok, not blood and guts, but poop and puke).
4. Inevitably they will take over the world. (Do the math)