Epidemiology: Definition, Uses and Approaches Epidemiology is the basic science of public health that deals with the study of the distribution and determinants of health-related states or events in specified populations and the application of this knowledge to control health problems. The term is derived from three Greek words: *epi* meaning “upon,” *demos* meaning “people,” and *logos* meaning “study.” Thus, epidemiology literally refers to the study of what happens to people in relation to health and disease. It is concerned not only with diseases but also with injuries, disabilities, nutritional conditions, environmental exposures, health behaviours, and other factors affecting the health of populations. Epidemiology differs from clinical medicine because its primary unit of concern is the population rather than the individual patient. Clinical medicine focuses mainly on diagnosis, treatment and care of individuals, whereas epidemiology investigates patterns of disease in groups and seeks to understand why particular health conditions occur more frequently in some populations than others. It therefore provides a scientific basis for disease prevention, health promotion and the planning and evaluation of health services. One of the most important uses of epidemiology is to describe the distribution and magnitude of health problems in a population. Epidemiologists measure the occurrence of diseases through indicators such as incidence, prevalence, mortality rates, morbidity rates, case-fatality rates and survival rates. These measures help determine how common a disease is and how it varies according to person, place and time. For example, epidemiological data can reveal whether a disease is more common among children or adults, whether it occurs more frequently in urban or rural areas, and whether its incidence is increasing or decreasing over time. Epidemiology is also used to identify the causes and risk factors of disease. Many diseases result from the interaction of genetic, biological, behavioural, environmental and social factors. Epidemiological studies help establish associations between exposures and health outcomes. For example, studies have demonstrated relationships between tobacco smoking and lung cancer, contaminated water and diarrhoeal diseases, and certain occupational exposures and respiratory disorders. Identifying risk factors allows public health authorities to develop appropriate preventive measures. Another major use is the investigation and control of disease outbreaks. When an unusual increase in cases of a disease occurs, epidemiologists investigate the outbreak to identify the source, mode of transmission and population at risk. The investigation generally involves confirming the diagnosis, identifying cases, describing cases according to person, place and time, developing hypotheses, testing those hypotheses and implementing control measures. This approach is particularly important for infectious diseases such as food-borne illnesses, influenza, dengue,