Zombies

Zombies are the driving force of the series, appearing as the primary antagonists in Mad World: Wormwood, Abandon All Hope, Mad World, and finally Mad World: A New Day. In addition to being called Zombies, they have been referred to as Ghouls, the Infected, and Walkers. As revealed in Mad World: Wormwood, Zombies are the product of an extraterrestrial parasite digging into the nervous system of a human host. The parasites reproduce at an incredibly fast rate prompting the urge to bite other individuals in hopes of spreading. In addition to the aforementioned, the parasites also cause an acceleration in the metabolism of the infected individual, which prompts them to resort to cannibalism otherwise they'll deteriorate to nothing.

The durability of the infected varies from person to person, depending entirely on the infectee's physical condition. Zombies also possess intelligence of some sort, demonstrated by their ability to use basic tools to achieve certain goals. It is believed that the undead share memories of their past life.