When experimental chemicals are introduced to the environment in an effort to fortify wildlife against pollutants, the effects are catastrophic.
The changes are nearly immediate, and many plants and wildlife become twisted into dangerous beasts.
People living in larger cities remain unaffected, and later wall themselves off from the rest of the world, while those closer to wilderness are eventually changed as well.

In the distant future, most smaller settlements have been consumed by the wilderness and the survivors have migrated into the relative safety of strongholds.

Those who harbor mutations have since been labelled "Unnaturals" for their visible oddities.

Carrion