Antisera prepared in animals were the first really effective therapeutic tools available to physicians for the management of active infections. However, it soon became apparent that the very phenomenon which produced immunity also produced an aberration of immunity: an allergic illness sometimes ensued, ranging in severity from a few benign skin lesions to rapidly fatal anaphylaxis or laryngeal edema. Thus, the first major therapy for control of active infections became a classic example of the two-edged sword, or iatrogenesis. A new item had been added to what Moser aptly called "diseases of medical progress." In the early part of this century, when relatively crude serum products were used in large amounts, as many as 80% of all patients developed some form of allergic reaction when treated with antisera prepared in animals. Then, as purified antisera preparations were developed, the incidence of allergic reactions dropped to an insignificant figure.