Abstract
Epidemiology is the study of the distribution and determinants of health-related states or events in specied populations and the application of this study to control health problems. It reflects that diseases are not randomly distributed in the population and that individuals have certain characteristics that predispose to, or protect against, a variety of different diseases. Disease is the result of an epidemiological triad, where disease results from the interaction of: the human host, an infectious agent or toxin, and the environment that promotes the exposure. This chapter focuses on the use of epidemiology to identify the source of diseases caused by microorganisms or toxins. Microorganisms are very efficient at infecting humans, using a number of different strategies and mechanisms. The deliberate dissemination of a biological agent by many of these mechanisms presents the latest challenge to public health. It requires a careful epidemiologic investigation to determine whether an outbreak of infectious disease is because of the intentional release of an agent, or is naturally occurring. A number of molecular techniques have been developed for sub typing bacteria, viruses, fungi, and protozoa, which will facilitate this investigation as well as identify clusters of related microorganisms.