The epidemiology of foodborne diseases in the United States have changed in recent decades as new pathogens have emerged, the food supply has changed, and the number of people with heightened susceptibility to foodborne diseases has increased. Emerging pathogens are those that have recently increased or are likely to increase within 2 decades. Emergency is often the consequence of changes in some aspect of the social environment. The global economy, for example, has facilitated the rapid transport of perishable foods, increasing the potential for exposure to foodborne pathogens from other parts of the world. Other factors altering foodborne disease patterns are the types of food that people eat, the sources of those foods, and the possible decline in public awareness of safe food preparation practices. Aging, extension of life expectancy for the chronically ill through medical technology, and the AIDS epidemic have increased the public health impact of foodborne diseases because they increase the proportion of the population susceptible to severe illness after infection with a foodborne pathogen. The evolving epidemiology of foodborne diseases must be monitored and understood to implement appropriate prevention technologies.