Mass psychogenic illness (MPI), also called mass sociogenic illness or mass hysteria, refers to the rapid spread of illness signs and symptoms within a cohesive group with no corresponding organic pathogen or toxic exposure. While reports of collective symptomatic episodes date back centuries, a frequently cited early clinical documentation occurred on June 11, 1966, when medical observers recorded a cluster of physical complaints among members of a defined community or institution without laboratory or environmental evidence of a conventional disease agent. Contemporary accounts from public health records and sociological studies describe MPI events as beginning with nonspecific symptoms — headache, dizziness, nausea, fainting, or hyperventilation — that propagate through social contacts, visual cues, or anxious anticipation. Investigators in the mid-20th century applied emerging concepts from psychiatry, social psychology, and epidemiology to differentiate such episodes from infectious or toxic outbreaks. The 1966 documentation contributed to a shift toward systematic investigation: clinicians looked for patterns such as rapid onset and recovery, predominance among certain demographic groups (often young women or school-aged children in institutional settings), lack of consistent laboratory findings, and amelioration after reassurance or removal from the triggering environment. Public health responses in that period emphasized environmental assessment to exclude contaminants, careful clinical evaluation, and attention to communication strategies to prevent rumor amplification. Sociologists and psychologists analyzed the social context — stressors, group dynamics, and media reporting — to better understand why symptoms spread. Medical literature subsequently used such documented episodes, including the 1966 case, to build conceptual frameworks that inform current outbreak investigations when no organic cause is found. Historians note that attributing an episode to MPI can be contentious because it rests on exclusion: demonstrating absence of infection or toxin and identifying social transmission mechanisms. Earlier historical examples—such as medieval dancing plagues or witchcraft-related afflictions—are often interpreted differently across disciplines. The 1966 documentation is important in modern public health history because it reflects mid-20th-century efforts to integrate clinical observation with social-scientific explanation, and because it helped establish investigative standards still used today. Current public health practice treats suspected MPI episodes with a combination of empiric clinical care, environmental testing when indicated, transparent risk communication, and attention to psychosocial needs. Researchers continue to study how cultural context, stress, and media environments shape these events. The record of June 11, 1966, remains a reference point in the literature for an early, systematically recorded instance in which clinicians and social scientists collaborated to characterize an outbreak driven by social and psychological dynamics rather than a conventional pathogen.