A deadly rash strikes scores of infants in France; children in a Connecticut town develop arthritis at an alarming rate; heart attack rates climb steadily over two decades and then drop. Mysteries like these are the challenges of epidemiologists, the scientists who isolate the causes of disease, track their modes of transmission, and measure the success of treatments. The authors of this text trace the history of epidemiology, describing the detective methods used to control the spread of contagious diseases and to investigate complex non-contagious diseases such as cancer, heart disease, and occupational illnesses.