In recent years, the growing problem of antibiotic resistance has enabled diseases once on the verge of eradication, such as tuberculosis, to reemerge as major threats to human health. The resurgence of old diseases, as well as the appearance of new ones, has prompted scientists to examine closely the factors that enable pathogens (disease-causing organisms) to invade and persist in human populations. Together, the problems of emerging infectious diseases and antibiotic resistance have fueled the development of the field known as evolutionary epidemiology. Fundamental to this new discipline is the recognition that pathogens are themselves biological entities that evolve in response to the challenges of their environment.