The Gastrea virus spreads like wildfire, causing each infected host to rapidly mutate, gaining new powers and abilities with every stage of development, even while they continue to attack, kill and infect multiple new hosts. Unable to fight a pandemic enemy that turns defenders into adversaries, the shocked remnants of the human race are forced to retreat into cities surrounded by giant monoliths made of Varanium, one of the few materials that can stop the Gastrea. (myanimelist.net)