The paper analises the incidence of nosocomial infections in Brazil and in the United States of America, emphasizing its letality rate when compaired with some classic communicable diseases. The paper also expiains the high incidence of nosocomial infections in industrialized countries, in spite of ali the sophistication of installations and equipments. It is pointed out that social and economic developments are able to produce a substantial decrease in the origin of exogenous diseases without impeding the increasing of the endogenous ones. Some evidences are shown to demonstrate the difficulty with the health personal substitute the magic rituais of prophylaxis by coherent administrative proceedings derived from the scientific knowledge. It is concluded that this historie process of rationalization will inevitably be reluetant and slow.