A crisis is emerging that could pose as grave a threat to public health as obesity or substance abuse: social isolation. Neuroscientists have identified regions of the brain that respond to loneliness, and a powerful body of research shows that lonely people are more likely to become ill, experience cognitive decline, and die early. Across the industrialized world, millions of people live with sparse human contact, putting their well-being at risk. Does social media drive loneliness, or help to cure it? How does loneliness alter the brain, and how can we treat this condition? Featured guests are Carla Perissinotto, Dixon Chibanda, Julianne Holt-Lunstad, and Katie Hafner.