Epidemiology is a field of study that looks at patterns of health and disease within a population. This includes the study of factors that contribute to illness. To determine the frequency and causes of illness, epidemiologists focus on studying communities rather than individuals. Use of epidemiology to study characteristics of illnesses and their associated factors can be used to prevent and control public health problems.

Epidemiologists study not only infectious diseases, but also environmental exposures to toxins and pollutants, workplace and crime-related injuries, birth defects, mental health, and substance abuse. To characterize these illnesses and conditions, epidemiology depends on statistics to measure rates of incidence, prevalence, and mortality.

Incidence is the number of new cases of a particular illness over a set period of time, while prevalence is the total number of existing cases. Mortality rate is the frequency of death within a population from a specific illness. Together, these rates characterize illness at the population level.

There are two main types of epidemiological studies: descriptive and analytic. Descriptive studies describe, or characterize, an illness. The types of people affected, their location, and the timing of onset or infection are examples of the types of descriptions important to epidemiology. Analytical studies, in contrast, analyze or test predictions and assumptions. Analytical studies help narrow the causes of and risk factors for illness. Both of these methods of data collection and interpretation can be used to learn more about the factors contributing to illness and how to predict, prevent, and combat them.