During the twentieth century, medical theory and practice underwent more profound changes than in all of the years since the time of Hippocrates (460?-377? B.C.). Since World War II, changes in science and society have transformed the theoretical, institutional, educational, economic, and ethical foundations of medicine. For example, research on the growth of stem cells has sparked both hope for revolutionary medical applications and profound ethical challenges. In 1999 the editors of the journal Science selected stem cell research as the "Breakthrough of the Year." Embryonic stem cells could be used to produce specific types of cells and tissues. Eventually, stem cells might be used to build new body parts to replace failing human organs.

Nineteenth-century scientists and physicians introduced the modern germ theory of disease and made it possible to identify the cause and means of transmission of many infectious diseases. Since 1950 many...