Epidemiology is a fundamental population science and tries to answer questions about health-related behaviors and outcomes in populations. Epidemiologists study how health and disease are distributed in populations and the factors that influence or determine those distributions. Why does a disease develop in some people and not others? Why are some people healthier than others? The premise underlying epidemiology is that disease and health-related factors are not randomly distributed in a population. Rather, each of us has certain characteristics, life situations and habits that predispose us, or do not predispose us, to states of health and illness.

The scope of epidemiology is described in the diagram below. It shows six components and the kinds of questions that can be asked. These components are the basis for study of a wide range of disease and health conditions, including infectious diseases, chronic diseases, injuries, and states of health and health behaviors. As the flow of the diagram shows, what is learned from epidemiology studies can often be used to model public health prevention and control programs; this is one reason why epidemiology is often said to be at the heart of public health.