An occasional employee of a local gas company, Albert Dadas suffered from a strange compulsion that led him to travel obsessively, often without identification, not knowing who he was or why he travelled. He became notorious for his extraordinary expeditions to such far-reaching spots as Algeria, Moscow and Constantinople. Medical reports of Dadas set off at the time a small epidemic of compulsive mad voyagers, the epicentre of which was Bordeaux, but which soon spread throughout France to Italy, Germany and Russia. Today we are similarly besieged by mental illnesses of the moment, such as chronic fatigue syndrome and attention deficit hyperactivity disorder. The debate rages about which of these conditions are affectations or cultural artefacts and which are real. In Mad Travellers, Ian Hacking uses the Dadas case to weigh the legitimacy of cultural influences versus physical symptoms in the diagnosis of psychiatric disorders. He argues that psychological symptoms find stable homes at a given place and time, in ecological niches where transient illnesses flourish. Using the records of Dadas's physician, Philippe Tissie, Hacking attempts to make sense of this strange epidemic. While telling his fascinating tale, he raises probing questions about the nature of mental disorders, the cultural repercussions of their diagnosis and the relevance of this century-old case study for today's overanalyzed society.