The World Ended A Long Time Ago... 

At least, the world we currently know ended a long time ago.  Somewhere at least four generations ago, humanity was lost to the end of times. A fungal infection, born in the blood, caused the dead to rise. They rose angry and hungry, and ravaged the creatures around them. The infection spread violently and rapidly. Quickly, the world fell into chaos. Countries far and wide attempted to slow the rampant spread of the infection.  Weapons were were fired, targeting both public ground and air travel to knock out means of mass transportation, in an attempt to prevent the pre-fall aircraft and ships from carrying the plague carrying infected hosts to places not yet touched. Humanity declared war on itself with the hopes of culling the plague before it could go any further.
 

HUMANITY FAILED

As humanity acted in desperation to eliminate the threat, the infection mutated and adapted to the weapons that were meant to destroy it. The plague began to warp the bodies of humanity into different forms of undead hunters.  The faster it spread, the longer it was hosted, the more it mutated. 

Humanity is long gone. 

WE ARE THE STRAINS OF LIFE 

Generations after the world had fallen and the Earth was a wasteland, mutated strains of humanity rose in the wake of what was the greatest civilization of mankind.  As the irradiated winds blew and the undead threats of the land continued to hunt, the living became mockeries of what life used to be.  While the bodies became stronger, able to shrug off all but the most grievous wounds, the flesh grew to be a new host for the parasitic entity known commonly as 'The Infection'.  

Inside every survivor strain is the roots of the Infection. It penetrated the Earth and those upon and caused all the be carriers. From the barely sentient Full dead to the hearty Red Star, each and every strain carries within them the truth that eventually all life will fall to the Gravemind - The Infection inside the Earth. All of the strains of life will eventually join the undead legions.  Mother or child, Priest or Prisoner, everyone ends as a monster.