If you were to ask any of the 1 million Americans living with multiple sclerosis, they’d probably say their disease started with changes so small they almost didn’t notice them: a wobbly step, a weakening grip, sight going soft around the edges. But MRI scans of their brains — dotted with ghostly white scars — would tell a different story. Those scars are signs of inflammation dating back multiple years. Each spot represents a dead zone filled with mangled remains of thousands, sometimes millions, of neurons. Like city blocks going dark during a power outage, these cells blinked out one...