2. A Modern Definition Study of the occurrence and distribution of health-related diseases or events in specified populations, including the study of the determinants influencing such states, and the application of this knowledge to control the health problem (Porta M, Last J, Greenland S. A Dictionary of Epidemiology, 2008)

3. Uses of Epidemiology To study the cause (or etiology) of disease(s), or conditions, disorders, disabilities, etc. –determine the primary agent responsible or ascertain causative factors –determine the characteristics of the agent or causative factors –define the mode of transmission –determine contributing factors –identify and determine geographic patterns

4. Uses of Epidemiology To determine, describe, and report on the natural course of disease, disability, injury, and death. To aid in the planning and development of health services and programs To provide administrative and planning data

5. The Epidemiology Triangle Time

6. Epidemiologic triad Host AgentEnvironment Biological agents Physical agents Chemical agents Nutrient agents Mechanical agents Social agents Physical environment Biological environment Social environment Demographic characteristics Biological characteristics Socioeconomic characteristics

7. The Epidemiology Triangle The agent is the cause of the disease Can be bacteria, virus, parasite, fungus, mold Chemicals (solvents), Radiation, heat, natural toxins (snake or spider venom) The host is an organism, usually human or animal, that harbors the disease Pathogen disease-causing microorganism or related substance Offers subsistence and lodging for a pathogen Level of immunity, genetic make-up, state of health, and overall fitness within the host can determine the effect of a disease organism can have upon it.

8. The Epidemiology Triangle The environment is the favorable surroundings and conditions external to the human or animal that cause or allow the disease or allow disease transmission Environmental factors can include the biological aspects as well as the social, cultural, and physical aspects of the environment Time accounts for incubation periods, life expectancy of the host or pathogen, duration of the course of illness or condition.

9. Endemic, Epidemic and Pandemic Endemic - The habitual presence (or usual occurrence) of a disease within a given geographic area Epidemic - The occurrence of an infectious disease clearly in excess of normal expectancy, and generated from a common or propagated source Pandemic - A worldwide epidemic affecting an exceptionally high proportion of the global population Number of Cases of Disease Time

10. Two Broad Types of Epidemiology: I descriptive epidemiology: examining the distribution of disease in a population, and observing the basic features of its distribution I analytic epidemiology: investigating a hypothesis about the cause of disease by studying how exposures relate to disease

11. Descriptive vs. Analytic Epidemiology Descriptive Descriptive Used when little is known about the disease Used when little is known about the disease Rely on preexisting data Rely on preexisting data Who, where, when Who, where, when Illustrates potential associations Illustrates potential associationsAnalytic Used when insight about various aspects of disease is available Used when insight about various aspects of disease is available Rely on development of new data Rely on development of new data Why Why Evaluates the causality of associations Evaluates the causality of associations Both are important!