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An introduction to epidemiology

Introduction to epidemiology and how it is used to support public health activities.
SPEAKER: In this presentation we introduce epidemiology through two bite-size learning outcomes. By the end of the presentation, you should be able to describe the definition of epidemiology, understand how epidemiology supports public health activities.
The word epidemiology comes from a Greek origin. Epi meaning upon. Demos meaning population. And logos meaning study. So if we put it together, this translates to a study of that which is visited upon a population. The definition that we now use is epidemiology is the study of distribution of health outcomes or disease in a population, determinants that influence the occurrence of disease, and application which provides direction for public health action.
The distribution of disease can be shown as the frequency and pattern of occurrence. Frequency can be a number or the rate of risk of disease in a population. The pattern of occurrence can be described by time, for example, an annual or a daily occurrence, or by place, for example, a rural or an urban location, by characteristics, for example, by age, gender, ethnicity.
Determinants, the risk factors or causes of occurrence of disease. These relate to exposure, behaviour, and genetic risk factors.
Application is the public health action that can be taken to improve health within a population. Application is also sometimes referred to as disease control strategies.
Epidemiology provides important information about a disease within a population and about what the risk factors are or could be. Health workers use this information to implement control strategies to prevent disease from occurring or spreading, to provide health system with priorities for planning and targeting health services, or to promote the use of evidence for effective clinical care and policy development.
In order to understand epidemiological information you need to be familiar with some commonly used terminology, such as prevalence and incidence. Prevalence is a measure of how many people have a disease, or a health state, in a given population at a specific time. A good analogy for this is a photograph of the population and knowing how many people in the photograph have a disease.
To calculate the prevalence of disease, we need to know two numbers. Firstly, the study population, that is the total number of people in the population we are studying. Then, we need to know the number of prevalent cases, that is the number of people with the disease at a specific time. To calculate the prevalence we divide the prevalent cases, the numerator, by the study population, the denominator. We can then multiply by 100 to get a percentage.
For example, let us consider a school with 50 children. And of those, five pupils wear spectacles. To calculate the prevalence we divide 5 by 50 and then multiply by 100 to get a percentage. This gives us a prevalence of 10% for spectacle wearing children at this school. Prevalence data allows us to understand and quantify the disease burden and health outcome. Health workers can use this information to allocate resources appropriately. They can also use it to measure the impact of health services by collecting prevalence data before and after a health intervention. For example, how much of a change did providing cataract surgery have on reducing blindness in a given population?
Prevalence information can be mapped to show where the need is the greatest. For example, as shown in this trachoma map, the areas where active trachoma is most prevalent will benefit from mass distribution of azithromycin.
Incidence refers to the number of individuals in a population who develop a disease over a specific time. This is a measure of risk, or rate that the disease will occur.
To calculate the incidence of disease we need to know two numbers. Firstly, the study population, the total number of people in the population we are studying. And then, the number of incident cases, that is the number of new cases that will develop the disease during a specific time period. To calculate incidence, we divide the number of new cases, the numerator, by the study population that was followed up over a specified period of time as the denominator.
In this example, we see how incidence can help medical professionals predict the probability of a complication developing. In our study population, we have 10,000 insulin dependent diabetics over the age of 40. They were followed up for a period of six years to see how many developed background retinopathy. 800 new cases are identified in that period of six years. To calculate the risk, or incidence, of developing retinopathy amongst diabetics in this population we take the number of new cases, 800, and divide by 10,000, the population being followed up. This gives us a risk of 0.08, or eight new cases in every 100 diabetics who would develop background changes in their retina in six years. This is called a probability.
In summary, epidemiology provides the clues for the magnitude, distributions, and determinants of diseases affecting a population. Prevalence data is very important for planning public health interventions.
Incidence data identifies the probability that a health outcome will occur.

The word ‘epidemiology’ is of Greek origin: ‘Epi’, meaning ‘upon’, ‘Demos’, meaning ‘population’, and ‘Logos’, meaning ‘study’. This translates to ‘a study of that which is visited upon a population’.

We now define epidemiology as the study of:

  • distribution of health outcomes or disease in a population
  • determinants that influence the occurrence of disease
  • application, which provides direction for public health action.

In this video step, we consider each of these components more thoroughly and discover some commonly used epidemiological terminology. While you’re watching, consider how epidemiological data supports, or could support, eye care work.

Book purchase

If you enjoyed this video you may wish to follow up your interest by purchasing “The Epidemiology of Eye Diseases, 3rd Edition” published by World Scientific. This book is a comprehensive coverage of the epidemiology of the world’s major blinding eye diseases, written by internationally acclaimed experts in each field, including several contributors to this course. Please note: The book is not essential to completing this course in any way.

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