As humans we are endlessly suggestible. We yawn when we see other people yawn, laugh when we hear others laugh and crave chocolate when we see other people eating chocolate. Likewise, when we hear about a medical complaint, there’s a natural inclination to check our own bodies for a similar twinge or itch. Taken to extremes, this tendency to ape others can create false illnesses in individuals and – most spectacularly – communities.

Neurologist Suzanne O’Sullivan has travelled the world visiting groups where ‘mystery’ illnesses that defy conventional medical explanation and treatment have emerged. In Sweden, children in refugee families have fallen into a sleep from which they cannot be woken for months or years. The condition, which has been named ‘resignation syndrome’, began in one family in the early 2000s, then spread to hundreds more children, all in the same situation. In Kazakhstan, people living in two neighbouring towns suddenly developed a similar tendency to fall asleep for days at a time, along with other strange symptoms including hyperactivity, vomiting and crying.

Sometimes these mysterious illnesses disproportionately affect teenage girls. On the Mosquito Coast in Nicaragua and Honduras, adolescent girls experience convulsions, tremors and hallucinations that sometimes compel them to bizarre behaviour like climbing trees and eating broken glass. The condition, known locally as ‘grisi siknis’ (or ‘crazy sickness’), has been documented