Killzone is a part of your brain that the Flare affects.
Killzone is the part of the brain affected by the Flare. This name is adopted after the Flare starts infecting people. The disease slowly eats away the Killzone, turning the infected (known as Cranks) into madmen. WICKED tries to use the patterns in the Killzone of those immune to the Flare to develop a cure, so they send a number of immune "subjects" into the Trials to obtain these patterns.
The drug known as the Bliss can limit the Killzone's activity. Since the Flare eats away at the brain faster when the brain works harder, the Bliss slows the disease down. Thomas' friend and fellow "subject," Newt, is not immune. Because of all the stress caused by the Trials, the Flare has eaten away at his Killzone much faster.