Paranoia by Craig DiLouie

Chad Carver, an orderly working at a mental health hospital, knows all about conspiracy theories thanks to some of his more paranoid patients-dark fantasies about black helicopters, thought control, international bankers and secret U.N. armies. Then his long-lost brother Palmer appears at his door claiming to be a renegade from the biggest secret society of all-the Illuminati-and predicts that the President will be assassinated in three days. After this prediction comes true, Chad plunges headfirst into Palmer's dark world where conspiracies are formed to fight conspiracies, where reality changes according to belief, and where even conspiracy theorists are agents of the Illuminati. When Chad learns that he must kill for his beliefs, the line between theory and reality disappears. Palmer tells him that the Illuminati are about to release a biological weapon, and if he kills the Grand Master of the Illuminati, the world will be saved. Or so the theory goes...