Infectious diseases vary greatly in the damage they have inflicted on human populations-smallpox, tuberculosis, and malaria have been much more destructive than acne or the common cold. This simple truth provides a basis for efforts to control emerging infectious diseases by emphasizing the need to distinguish emerging infectious diseases according to the threat that they pose to humans. Critical to this assessment is the breadth of diseases considered. If the breadth is too narrow the greatest infectious threats might fall outside the scope of inquiry and thus fail to be recognized and controlled. This concern draws attention to the value of defining emerging infectious diseases broadly to include diseases that (i) have long been established in human populations but have recently spread from one human population into another, (ii) are newly introduced into humans from other species (zoonoses), and (iii) have long been present in human populations but are newly recognized as infectious.