We live in worrisome times. Perhaps chief among these worries is a concern about health. Newly discovered diseases, ranging from Legionnaires' disease and Lyme disease all the way to AIDS, are reported regularly in the press. At the same time, expensive new technological devices, such as the CAT scan or the PET scan, are trumpeted urgently as ways of diagnosing and warding off otherwise hidden but possibly fatal conditions. New medical treatments,...