The zombie virus requires living cells to advance itself, becoming steadily more advanced and adaptable as the zombie devours more living tissue and learns more from it's environment. The virus is contained within the venom of zombies; specialised sacks develope, which when given enough time will inject the mutagenic virus into the victim.

The average normal zombie is little better than a mindless drone, only the most basic of brain functions left within their shambling shell of a corpse. Basic motor function and the need to eat are the only manifestations of intelligence within these zombies, a rudimentary herd-instinct forming them into hordes to better gain their food.

But, through a combination of the correct food supply and random chance, the virus can mutate within the zombie and cause a number of changes, 'evolving' the mindless drone into a higher form, which manifests itself in a number of ways.

MetaZombies - Stronger and faster than the average human, these zombies display heightened senses and physical abilities at the loss of durability, requiring less fire to bring down, and having to feed more often.

NecroZombies - Necrotic flesh hangs from these zombies, while not much faster nor stronger than the average human they require more fire power to bring down than other zombies, their decaying frames rapidly regenerating what little there is left.

BorosZombies - Reduced to a festering liquid-esque state by the zombie plague, these zombies require a host to survive, wrapping their putrid self around the host and absorbing into their body. When bonded to a host, the zombie displays increased speed, strength, endurance and senses, but an incredibly aversion to fire. Open flames terrify these zombies, their foetid essence generating flamable fumes. Without a host these zombies are able to move around in a snake-like fashion, stronger specimens able to form themselves into rudimentary skeleton-like structures for short periods of time, for better movement.

MutaZombies - "You are what you eat." has never been more true for these zombies; their unique mutation of the virus causing them to gain the traits of what ever they devour. Typically not as sturdy as other zombies, they often become more specialised due to their mutation, developing wolf-like claws or fangs, increased speed or strength. This variation is the only kind of zombie able to use firearms, by restricting their food source to humans they are able to increase their intelligence enough to use them.