Epidemiology as a scientific discipline and epidemiologists as practitioners are not clearly defined in the minds of most members of the healing arts. The etymology of the word (epi, upon; demos, the people) perhaps says it all—the study of how, where, when, and why a disease distributes itself over time within a population. It rests less on a body of knowledge and more on a way of thinking.
Epidemiology had its beginnings from or in close association with medicine. The acute infectious and transmissible diseases were the first to be recognized as being due to a foreign agent. They were also demonstrated to be controllable by interrupting, in some way, the passage of an offending agent from a diseased person to another susceptible person with or without knowing the specific nature of the agent. The logic involved in this approach evolved into the epidemiologic triad of the agent, the vector