In the 19th century, Joseph Lister related the germ theory of fermentation to the cause of putrefaction in wounds. Listerism was adopted because its success was greater and more consistent than other methods of healing the sick. The circumstances which made this possible were a theory for explaining the scientific evidence, and a courageous person like Joseph Lister who was capable of bringing about the necessary changes. This study records how with much pain and trial and error the prevention of nosocomial infections was achieved in the 19th century. Today, we have learned we must implement again Lister's prevention techniques and other precautions in our hospitals to prevent the spread of nosocomial infections. Illus.