As others have pointed out the similarities to I Am Legend in the chapter discussion thread for 23-2, I also took this idea and concept from Stephen King's Cell, where people get their brains altered by "The Pulse" but the first batch that received it got a bad dose. So when these people "turned" they were psychotic maniacs, but later on when a perfected pulse was unleashed those that turned were calmer, more intelligent, not nearly as blood thirsty, but increasingly more manipulative.

Those guys also had weird psychic powers. I mean, it's to be expected in a Stephen King book after all. It just seems the further and further we get away from the initial outbreak, the zombies of WA are evolving into radically different creatures than what we originally encountered.