After Earth

A range of man-made corruptions have destroyed Earth after humans failed to stop their own creations. After almost 90% of the world's population perished from extreme weather conditions, pollution and rapid degrade of the Earth's surface, a deadly organism appears out of nowhere and inhibits an unsuspecting human as it's host. It destroys it's host's DNA, causing almost immedient genetic mutation. First it changes the brains chemical make up, practically making the human devolve back into a mindless, aggressive animal. Then the body changes, quickly adapting to its host to become the perfect hunter. To ensure that the organism survives, the mutated host infects another by using special poison that runs throughout the victims body.

Soon the Earth is overrun by these "Hunters". The few remaining humans fight to survive in the world where monsters constantly hunt for your blood and flesh. A world where your best friend, your brother or even your parents, turn into a bloodthirsty monster that would kill you without a second thought.
Now in the world they once called home, humans fight with only one mission at hand; kill or be killed.
But no matter how hard you try, it's only a matter of time before you join Decay.
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(The cover is not mine, all credit goes to the original author :p)