What if the digital traces left by our searches could show doctors and medical researchers something new and interesting? What if the data generated by our searches could reveal information about health that would be difficult to gather in other ways? In this book, I argue that Internet data could change the way medical research is done, supplementing traditional tools to provide insights not otherwise available. I describe how studies of Internet searches have, among other things, already helped researchers track to side effects of prescription drugs, to understand the information needs of cancer patients and their families, and to recognize some of the causes of anorexia. I show that the information collected can benefit humanity without sacrificing individual privacy. I explain why people go to the Internet with health questions; for one thing, it seems to be a safe place to ask anonymously about such matters as obesity, sex, and pregnancy. I describe in detrimental effects of "pro-anorexia" online content; tell how computer scientists can scour search engine data to improve public health by, for example, identifying risk factors for disease and centers of contagion; and tell how analyses of how people deal with upsetting diagnoses help doctors to treat patients and patients to understand their conditions.