Pneumocystis jirovecii is a fungal infection that causes pneumonia, often in highly immunocompromised patients, such as those recently undergoing transplants. Before 2019, Marwan Azar, MD, FAST, and his colleagues in Internal Medicine and the Section of Infectious Diseases had not witnessed an outbreak of Pneumocystis pneumonia at Yale New Haven Hospital (YNHH). Despite genetically confirmed outbreaks in many areas, including Canada, Europe, and Japan, there had never been such a case in the United States.