Sometimes we feel so deeply that it causes physical discomfort, a visceral reaction that transports emotion to the body’s core. Immense joy takes the breath away; an awful sight induces nausea. An intense form of this relationship between mind and body is called “conversion disorder,” in which stress or depression manifest as convulsing limbs, blindness, paralysis. The emotional condition converts into a physical one. Physicians used to call this strange psychologically driven illness “hysteria.” Stranger still is mass hysteria, in which a group of people sicken from the same emotional trigger.

Imagine, as you ride the bus, a passenger steps off muttering threateningly. You smell something strange and catch the eye of another passenger who wrinkles her nose, frowning. Minutes ...