The science of epidemiology has been described in numerous ways. John M. Last's description, in A Dictionary of Epidemiology,* is perhaps the most frequently cited:
"The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems."
A more simple description comes from Donald F. Austin and S. Benson Werner, in their Epidemiology for the Health Sciences,** published in 1974:
". the study of how and why diseases are distributed in the population the way they are . in other words, why some get sick and some don't."
Most descriptions mention, in one way or the other, the following seven ideas: methods, frequency, distribution, determinants, disease, populations, and prevention. Therefore, epidemiology consists of the methods used to study the frequency, distribution, and determinants of disease in populations, so that the disease can be prevented.
* Last, John M., Editor, (2001). A Dictionary of Epidemiology, 4th Edition. New York: Oxford University Press.
** Austin, Donald F. and Werner, S. Benson (1974). Epidemiology for the Health Sciences. Springfield, Illinois: Charles C. Thomas.