What is Epidemiology?

Epidemiology is the study of the frequency, distribution, and determinants of diseases and other health-related states and events, in specified populations and population groups within a common context, from the local level (e.g. neighborhoods, schools, communities) to the regional, national and global level). In other words, Epidemiology is the study of how often diseases occur and how they are patterned in different groups of people and the investigation for identifying the determinants (e.g. risk and protective factors) influencing these processes which may be biological, genetic, behavioural, social, cultural, economic and/or political.

In practical terms, Epidemiological research helps us understand how many people are affected by a disease or other health-related outcomes in a population, whether those numbers are changing, and what impact this health-related outcome has in public health and generally the society. The ultimate aim of Epidemiology is to provide relevant and timely data, findings, and conclusions to enable disease prevention, health promotion and the reduction of health inequalities.

Epidemiology is an applied science and the fundamental quantitative tool of Public Health. Understanding trends, patterns and determinants of disease and health becomes meaningful only in the context of effective utilization of epidemiological findings to plan, act, implement and evaluate relevant Public Health measures to control and address the issue.

The field of Epidemiology is further divided into different sub-fields (not necessarily mutually exclusive). Some of the main ones are: Chronic Disease Epidemiology, Infectious Disease Epidemiology, Environmental Epidemiology, Social Epidemiology, and Genetic Epidemiology.

Research in the sub-filed of chronic disease epidemiology focuses on the frequency, distribution, natural history, and aetiology of chronic diseases (such as cancer, cardiovascular disease, obesity, type 2 diabetes, dementia, and others) and their determinants (diet, physical activity, smoking, alcohol consumption, and others) at the population level. Chronic disease epidemiology provides valuable data for the prevention of chronic disease in populations by identifying the main determinants at different levels (individual, societal, environmental). Ultimately, chronic disease epidemiology aims at enabling health promotion for achieving longevity, good health, and high quality of life for all.

The sub-field of infectious disease epidemiology focuses on identifying how and why infectious diseases emerge and spread among different populations, and what strategies can prevent or contain the spread of disease. This sub-field studies the complex relationships between hosts and infectious agents, focusing on aspects such as disease transmission dynamics, as well as evidence-based approaches for preventing or containing infectious diseases in populations, including management of epidemics and pandemics. Ultimately, infectious disease epidemiology aims to minimize the impact of pathogens on public health.

The sub-field of environmental epidemiology focuses on the investigation of health effects related to chemical, physical, and biological hazards in the context of the natural, built, and social environment, in which people live and work. This sub-field also focuses on the alarming public health threat of climate change, highlighted as the number one threat to global health by the WHO. Environmental epidemiology also includes the important sub-fields of occupational health and food safety.

This sub-field of epidemiology focuses specifically on the social determinants of health, that is the sociostructural conditions in which people are born, grow, live, work and age. Factors of interest in social epidemiology include poverty and material circumstances, educational attainment, area of residence, and ethnic background. In the core of social epidemiology is that the distribution of advantages and disadvantages in a society reflects the distribution of health-related outcomes in a population, the so-called social patterning of health and disease, subsequently leading to health inequalities. Ultimately, social epidemiology aims to generate valid and timely data that will help in narrowing the socioeconomic gap in health within and between societies and achieve equal opportunities and a good health status for all people globally.

This rapidly expanding sub-field of epidemiology emerged after the huge scientific revolution of the decoding of the human genome in the early 2000s. This scientific breakthrough enabled the investigation of the genetic determinants of disease, both as regards the identification of genetic mutations solely responsible for causing, usually rare, genetic (monogenic) diseases, as well as the more frequent genetic predisposition to complex diseases, involving several different genes and polymorphisms. Genetic Epidemiology also investigates how genetic factors interplay with environmental factors (gene-environment interactions), providing thus a more complete picture of disease aetiology. The ultimate aim of genetic epidemiology is to enable more focused and tailored public health interventions in specific population groups, an approach referred to as precision public health.