Do not ignore Intelligence. Most new players spam the "Bite" button, turning into a fast but brainless ghoul. By level 15, you will hit a wall where humans use shotguns. You need brains, but you need strategy to get them.
The mechanic unlocks at Level 40. You must assign your converted zombies to different tasks. A Zombie Life Walkthrough
Go forth, you magnificent rotting corpse. The world is your buffet. Do not ignore Intelligence