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 plague of the living dead scourge came up from the ground, the living quickly fell and joined the ranks of the enemy, and society as we know it was purged from the planet. 

While this blood born fungal infection caused the dead to rise countries attempted to slow the rampant spread of the infection.  Weapons were erupted on both the ground and in the air to knock out means of mass transportation, to prevent the pre-fall aircraft and ships from carrying the plague carrying infection hosts to places not yet touched, and 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 blood born fungal infection mutated, changed, and adapted to the weapons that were thrown at it.  The plague began to adapt the bodies of humanity into different forms of undead hunters.  As the plague spread, the reaction that humanity had to the infection changed. 

Humanity is long gone. 

We are the strains of life 

Generations after the world has fallen, after modern civilization was lost, the 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.  From the barely sentient Full dead to the hearty Red Star each and every strain carries within them the truth that eventually all of the strains of life will fall to the grave mind.  All of the strains of life will join the undead legions.  Everything can, and does, still get worse.