Encephalitis lethargica, also known as epidemic encephalitis, emerged as a new infectious disease near the end of the First World War. Bacteriologic, epidemiologic, and clinical investigation produced no clear consensus regarding the nature of the disease, even as several other experimentally demonstrable "encephalitides" appeared on the scene. By 1940, new encephalitis lethargica cases had almost entirely disappeared, and neurologists renamed this once-novel infection as an amorphous syndrome of marginal interest. A variety of forces influencing the fate of encephalitis lethargica's epidemic status can be seen at work in the Matheson Commission, whose members hoped to use encephalitis as a model disease that might supplant their reliance on clinical phenomenology with a causal analysis of nervous disease grounded in the laboratory. When it failed to live up to these expectations, the model was abandoned. Epidemic encephalitis was soon forgotten.