There is a general impression that treatment of a disease in which the etiologic agent is a virus is of no value after the infection has become established. Rivers1probably presented the opinion of other authorities as well as his own when he wrote the following in 1936: "Such a phenomenon throws light on the clinical observations that the administration of convalescent or immune serum to an individual in whom the signs and symptoms of a virus disease are already manifest is without value." It is of interest that Rivers2has indicated (in the Lane lectures given by him in 1939) that he has modified his point of view to the extent of admitting that measles can be treated after the onset of the typical early signs and symptoms of the disease. However, the majority of clinicians and investigators hold to the view expressed by this authority in