It is a type of public health science. People who work in this field are known as epidemiologists, sometimes called health detectives. They investigate how diseases are spread, where they come from, and how to stop them. 

Epidemiology is the foundational (basic or core) science of public health. It is the study of the distribution (frequency and patterns) and determinants (risk factors or causes) of a health condition or outcome in a group of people.  

Epidemiological data and research help us gather key information about health conditions and outcomes, including:

• Who is most at risk in terms of individual characteristics (age, sex, etc.), behavioral, and other ‘risk’ factors
• What the features of the outcome are (symptoms, frequency of cases, impacts)
• Where and when the health condition is occurring
• Why and how the health condition develops or spreads  

This information is then used to create ways to prevent, treat, and manage health conditions.  

Example: Epidemiological research shows that young children and older adults are most at risk of getting the flu during the colder months. This is why young children and older adults should get their flu vaccine before the colder months.

In epidemiological research, the population is the group of people we want to study for a health issue and draw conclusions about. Populations can be very broad or very specific, depending on the health outcome and research question being studied. 

Examples of populations include:

• United States adults 18 or older
• Men ages 18-35 living in Albany County, NY from 1993-2000
• New York State school-aged children 

It is often very difficult, time-consuming, and expensive to study and collect information from every member of the population.  A sample refers to the smaller group from the overall population who we are able to study or collect information about. Researchers do their best to make sure the sample is as similar to the population as possible. 

Descriptive epidemiology focuses on describing the patterns of a health outcome. It can show how a health outcome varies by person, place, and time. In other words, which factors are associated with the outcome, but it does not necessarily explain the cause(s) of the outcome.

Etiologic epidemiology focuses on the possible cause(s) of a health outcome. Etiologic is pronounced "ee-tee-uh-LAJ-ik."

In the next section, we explore in a little more depth the difference between ‘cause’ and ‘association.’

In epidemiology, we often want to understand health outcomes (cases) in a population and the factors that may cause or be associated with them (exposures).

Cases refer to individuals who have a particular health outcome that we are interested in studying. This includes a disease, condition, or other health outcome of interest. For example, received a vaccine, enrolled in a health program, went to the emergency department.  

Exposures refer to factors or events that may be associated with a health outcome. These include, but are not limited to: individual characteristics, behaviors, medical treatments, environmental conditions, or other factors that may make someone more or less likely to experience the health outcome.

Example: Let’s say we are interested in identifying whether people who smoke are more likely to be diagnosed with lung cancer. In this example, the exposure is the behavior of smoking, and cases would be defined as those who are diagnosed with lung cancer. 

Causality shows that one event, or exposure, causes an event that follows it. The event that follows the exposure is called the "outcome."  This means if the cause had not been present or hadn’t taken place, the outcome would not have occurred.

Association shows that there is a relationship between two events. But just because there is an association between two events, does not mean that one event causes the other. Sometimes there is an underlying issue that links both events. Public Health scientists refer to this issue as a confounder.

Example: If we look at the number of sunburns and the amount of ice cream sold every day for an entire year, we will likely find an association. But we will not find a causal relationship between the two. 

• On days with higher ice cream sales, there are also a lot of sunburns--an association.
• Clearly ice cream sales do not cause sunburns, and sunburns do not cause ice cream sales to go up.
• In this example, the reality is that a third factor (hot and sunny weather) is the cause of both outcomes. The hot and sunny weather causes people to buy more ice cream and causes more sunburns.