The Killzone was the part of the brain affected by the Flare. This name was adopted after the Flare started infecting people and turning them violent. The disease slowly ate away at the Killzone, turning the infected (called Cranks) into madmen and homicidal monsters. WICKED mapped the brain patterns and reactions in the Killzone of those Immune to the Flare in order to develop a Cure, so they sent a number of "Subjects" into the Trials to obtain these patterns.
However, there was a counter. A drug known as the Bliss could limit the Killzone's activity. Since the Flare ate away at the brain faster than when the brain works harder, the Bliss slowed down the disease. If a person, for example, had a lot of stress, the Flare would eat away at the Killzone much faster. It's often quite easy to spot someone using the bliss and many cities had a punishment for using it.