What disease did children start dying from in Chapter 1, and when did Ruby see the first child die in front of her?

Children began to die from a disease first called Everhart's Disease and then later Idipoathic Adolescent Acute Neurodegeneration (IAAN). The first child Ruby saw die was a girl named Grace in her fourth grade class. Grace was eating lunch. Then she dropped her food, her eyes rolled back, and she let out a small sigh. Then she collapsed dead.