"The plague I created eats away at the blood cells in the boys streams rapidly to the point where there is little oxygen flow to his brain. Because of this, the body changes and evolves into one of my creations, a feral. It seeks blood from outside sources to sate its own blood. I cannot guarantee your brother will not feel pain in the process that is long and annoyingly tedious to explain...but he won't be converted into a mindless beast either."