Bonuses

Overview
Bonuses (aka Upgrades or Perks) are special abilities/effects that are granted to the player whenever he reaches 100 killed zombies. Once he kills 100 zombies, he is granted the option to get one of the many bonuses a player can get, which are listed by level down below. The effects only stacks up to 3. Once you get to 3, you can't upgrade the same bonus anymore.