Epidemiology is the study and analysis of the spread (who, when, and where), patterns, and causes of health and disease in a defined population.

It is one of the most important parts of public health. It helps shape policy decisions and evidence-based practise by figuring out what causes diseases and who needs preventive care. Epidemiologists help design studies, collect data, and analyse it statistically. They also help change how the results are interpreted and get the word out (including peer review and occasional systematic review). Epidemiology has helped develop methods used in clinical research, public health studies, and, to a lesser extent, basic research in the biological sciences.

Disease causation, disease transmission, outbreak investigation, disease surveillance, environmental epidemiology, forensic epidemiology, occupational epidemiology, screening, biomonitoring, and comparing the effects of treatments, like in clinical trials, are some of the most important things that epidemiologists study. Epidemiologists use other scientific fields like biology to learn more about how diseases work, statistics to make good use of the data and come to the right conclusions, social sciences to learn more about close and far causes, and engineering to figure out how much exposure a person has had.

Epidemiology means “the study of what is on the people.” It comes from the Greek words epi, which means “on,” demos, which means “people,” and logos, which means “study, word, discourse,” which suggests that it only applies to human populations. But the word is often used to talk about studies of animal populations (called “veterinary epidemiology”); the term “epizoology” is also used, and it has been used to talk about studies of plant populations as well (botanical or plant disease epidemiology).

Hippocrates was the first person to tell the difference between diseases that “visit” a population (epidemics) and those that “live within” a population (endemics) (endemic). Epidemiology seems to have been used for the first time to talk about the study of epidemics in 1802 by the Spanish doctor Villalba in his book Epidemiologa Espaola. Epidemiologists also study what happens when two or more diseases spread through a population at the same time. This is called a syndemic.

Epidemiology is now used to describe and figure out what causes not only epidemic and infectious diseases, but also diseases in general and conditions that are related to them. Epidemiology looks at things like high blood pressure, mental illness, and obesity, among other things. So, this type of epidemiology is based on how the pattern of the disease changes how people work.