As Ruth McKernan’s father slowly succumbs to a mysterious illness, she relies on her professional training as a neuroscientist to help her work through her fear and grief. This moving memoir of love, science, and a parent’s death is an inspired blend of personal revelations and professional insights. Science is just one way of looking at life. As a neuroscientist working at the forefront of medical research, it is Ruth McKernan’s way. For a while it was her father Billy’s way, too. Indeed, science was a language they shared—until Billy inexplicably lapsed into a coma after being admitted to the hospital. As Ruth watches her father’s life ebbing away, she struggles to understand what is happening, grasping for control of the scientific knowledge that would allow her to objectify and analyze his medical condition. A postman’s son who parlayed his formidable intellect and natural strength of character into a successful career, Billy was always a difficult and demanding father. But it took his collapse and slow slide toward death for Ruth to realize how intense her feelings were for him. Ruth recounts the story of Billy’s last year while exploring a collection of cutting-edge scientific themes—delving into memory, consciousness, microbes, and stem cells—gracefully linking them together like pearls strung upon the thread of her father’s life. A true labor of love, Billy’s Halo shines with heartfelt emotion, yet manages to provide a crystal-clear explanation of the way our brains and bodies work in sickness and in health.