Trauma is contagious; its powerful affect and frequently unformulated memories can be transmitted—sometimes nonverbally and often mysteriously—within families, across generations, and from patient to clinician; in the latter case it is commonly referred to as vicarious trauma. In emphasizing trauma’s contagious quality, the current paper explores the relationship between vicarious trauma and dissociation. The author reviews a number of neuropsychological, cognitive, and psychodynamic explanations for this uncanny clinical phenomenon and its frequently unremarked boundary failures. Despite its disorienting effect on clinicians, it is suggested that in a long-term clinical engagement with a survivor of massive psychic trauma, vicarious trauma can be essential in helping both clinicians and patients process the traumatic material. Contrasting examples illustrate those occasions in which the emotional contagion can be addressed in treatment using containment, validation, and imagination, and other occasions, when there is no therapeutic contract, and other means must be found to formulate the dissociated material.