Epidemiology is the method used to find the causes of health outcomes and diseases in populations. In epidemiology, the patient is the community, and individuals are viewed collectively. By definition, It is the study (scientific, systematic, and data-driven) of the distribution (frequency, pattern) and determinants (causes, risk factors) of health-related states and events (not just diseases) in specified populations (neighborhood, school, city, state, country, global). It is also the application of this study to the control of health problems
Major areas of epidemiological study include disease causation, transmission, outbreak investigation, disease surveillance, environmental epidemiology, forensic epidemiology, occupational epidemiology, screening, biomonitoring, and comparisons of treatment effects such as in clinical trials. Epidemiologists rely on other scientific disciplines like biology to better understand disease processes, statistics to make efficient use of the data and draw appropriate conclusions, social sciences to better understand proximate and distal causes, and engineering for exposure assessment.
Epidemiology, literally meaning “the study of what is upon the people”, is derived from Greek epi ‘upon, among’, demos ‘people, district’, and logos ‘study, word, discourse’, suggesting that it applies only to human populations. However, the term is widely used in studies of zoological populations, although the term “epizoology” is available, and it has also been applied to studies of plant populations (botanical or plant disease epidemiology).
The distinction between “epidemic” and “endemic” was first drawn by Hippocrates, to distinguish between diseases that are “visited upon” a population (epidemic) from those that “reside within” a population (endemic). The term “epidemiology” appears to have first been used to describe the study of epidemics in 1802 by the Spanish physician Villalba in Epidemiología Española. Epidemiologists also study the interaction of diseases in a population, a condition known as a syndemic.
The term is now widely applied to cover the description and causation of not only epidemic disease, but of disease in general, and even many non-disease, health-related conditions, such as high blood pressure, depression, and obesity. Therefore, this epidemiology is based upon how the pattern of the disease causes a change in the function of human beings.