Through the first half of the twentieth century, tens of thousands of children in the United States were subject to high-dose thymic irradiation as part of a routine medical treatment meant to prevent sudden infant and childhood death. While rational in its impetus, excessive fear and panic served as a driving force in the standardization of this treatment. Facilitated by mass media and changes to medical malpractice law due to the growing predictive power of modern medical technologies, this fear was amplified through a feedback loop between physicians and parents. As a whole, pediatric prophylactic thymic irradiation complicates traditional models of medical reversal by introducing a case fear, and not only evidentiary practices, drive the reversal. Furthermore, the history of thymic irradiation provides a valuable case-study for a nuanced analysis of the role of fear in shaping medical decision-making and therapeutic practice beyond infectious disease. Lastly, the history of thymic irradiation offers timely insights into the relationship between fear, collectivism, and trust in medical authority which have implications for contemporary medical practice.