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Epidemiology

1. 1.1 What is epidemiology?
2. 1.1.1 Definition
3. 1.1.2 Health outcomes
4. 1.1.3 Frequency measures
5. 1.1.4 Explanatory factors
6. 1.1.5 Epidemiological function
7. 1.1.6 The empirical cycle
8. 1.2 Developments in epidemiology
9. 1.2.1 Historical examples
10. 1.2.2 Epidemiology today
11. 1.3 Developments in the discipline itself
    1. Recommended reading
    2. Knowledge assessment questions

1.1 What is epidemiology?

1.1.1 Definition

When a new infectious disease (such as COVID-19) is discovered, epidemiologists work with other scientists to find out who has it, why they have it, and what can be done about it. Epidemiology is the study of the distribution (frequency, pattern), and explanatory factors (causes, risk factors) of health-related states and events (health outcomes) in specified populations at a particular time. Using a COVID-19 outbreak as an example, epidemiologists identified the source of the outbreak by conducting field investigations in the community and health facilities in Wuhan, China. They collected specimens from the nose and throat for lab analyses. These field investigations informed those who were infected, when they became sick, and where they had been just before they became sick, which ultimately led to the identification of the likely source. Epidemiologists monitor and track the disease by using surveillance systems that report data on new cases of infection, symptoms, hospitalizations, treatments, deaths, and demographic information. Epidemiologists further investigate the disease to find out how contagious COVID-19 is, how to test for the presence of infection, what risk factors for severe illness are, what the natural and treated course of the disease on average is, and which treatments are most effective. Finally, epidemiologists contribute to developing guidance for preventive actions (e.g. face-mask wearing and social distancing) to slow the spread of the disease and lessen its impact.

Following on from the above, we can characterize epidemiology as follows:

• Health outcomes are the main focus of epidemiological research.
• Epidemiology is the study of health outcomes in human populations.
• Epidemiologists usually look at health outcomes in relation to other phenomena. These are factors suspected to have influenced the development of the health outcome in question (etiological factors), to give an indication of the presence of a particular health outcome (diagnostic factors), to be associated with the course of the health outcome (prognostic factors), or to mitigate the impact of the health outcome (interventions).
• The application of epidemiological study findings is meant to improve the wellbeing of the population.

Case 1.1 Scurvy and citrus fruits

James Lind, a Scottish ship's doctor, carried out an inventive experiment on scurvy patients in 1747. Scurvy (or scorbutus) is a disease characterized by bleeding gums, internal bleeding, stiff limbs and rough skin, among other things. It was very common in those days among crews of ocean-going ships. Long after Lind made his observations, it was found that scurvy is caused primarily by a deficiency of vitamin C (ascorbic acid), which is needed to synthesize collagen, a substance that strengthens blood vessels. James Lind selected 12 cases in the same stage of the disease on his ship. In addition to the common diet all participants received as their primary source of nutrition, he compared six "treatments": two patients were prescribed cider, two elixir of vitriol, two sea water, two a mixture of nutmeg and a commonly used medicine based on garlic and mustard seed, and two were given two oranges and one lemon a day. Only the patients who were given the orange and lemon treatment experienced a rapid cure. The mechanism by which scurvy can be cured and prevented using citrus fruit was not known at that time. However, it did not present an obstacle to taking effective measures. It took many years for citrus fruit to be introduced as a prophylactic for this dreaded disease on ships. Nearly 50 years later (1 year after Lind's death), the Admiralty of England made issue of lemon juice compulsory in its navy. As a nod to Lind's pioneering work, the Institute of Naval Medicine includes a lemon tree in its crest.

1.1.2 Health outcomes

The basic variable in epidemiology, and in any epidemiological study, is a health outcome. Epidemiologists are interested in the extent to which diseases or more generally health outcomes occur (i.e. their frequency) among the population. This defines the object of epidemiological research, namely health outcomes in human populations. The word "disease" needs to be interpreted broadly in this context: it can mean a broad variety of medical conditions, both infectious and non-infectious, acute and chronic, somatic and mental. In this context "health outcome" can refer to all sorts of phenomena on a continuum from full health to death from a condition. It can also refer to a disability, injury due to trauma, quality of life, or a physiological measure. As there is no term that encompasses all these aspects, we generally use the phrase "health outcome" - or simply "outcome" - in this book. Chapter 2 provides more detail on measuring various aspects of health. For the purpose of epidemiological research, we need to define the health outcome of interest as precisely as possible so as to enable us to detect relationships with explanatory factors.

1.1.3 Frequency measures

Epidemiology is the study of health outcomes and explanatory factors in humans. Animal experiments and observations in cell or organ cultures do not fall into the domain of epidemiology. Although the measurements are usually carried out on individuals, the results of epidemiological research always relate to groups of people. As the disease frequency for a group can be interpreted as the mean disease risk for each member of the group, the knowledge gained from epidemiological research is valuable not only at the group level (for public health) but also at the individual level (in healthcare). Epidemiologists ascertain whether each individual has the health outcome and then count the number of individuals with the health outcome in the group as a whole, thus yielding the epidemiological fraction.

This fraction provides the basis for all epidemiological measures of frequency (see Chapter 2). For example, we refer to incidence when counting new cases of a disease in a group at risk of contracting that disease (the at-risk population). Prevalence, on the other hand, relates to the number of existing cases of the disease. Incidence is also a measure of risk: it indicates the average risk of developing the health outcome during the specified period.

Health and disease are not equally distributed among the population; and it is this fact that lends epidemiology its raison d'être. The purpose of epidemiology is first and foremost to identify differences in health outcomes between populations. The distribution of a health outcome among the population becomes clear when we investigate the differences in frequency between groups of people at different times, in different places, and with different characteristics. Differences in time can manifest themselves between seasons or over years or decades. Differences between geographical areas can relate to continents, countries, regions of a country, urban versus rural areas, or districts of a town or city. Examples of individual characteristics that can be associated with differences in disease frequency are age, sex, genetic predisposition, occupation, and lifestyle characteristics such as smoking, drinking, and physical activities. Categorizing the population into subpopulations based on time, place, or individual characteristics gives us an understanding of the distribution of the disease risk, and enables us to identify risk periods, risk areas, or at-risk groups. Identifying the distribution pattern of a health outcome falls under the domain of descriptive epidemiology. The branch of epidemiology that aims to identify causal factors underlying the distribution of a health outcome is known as analytical epidemiology.

1.1.4 Explanatory factors

In addition to the health outcomes, epidemiology studies the factors related to the occurrence of health outcomes. These factors fall into three categories: etiological, diagnostic, and prognostic. The term "explanatory factors" is used in this book as a generic term for these factors. Individuals can be exposed to several factors either at the same time or successively. Epidemiologists are interested first and foremost in factors that are causally responsible or co-responsible for the development of a disease (etiological factors or risk factors) or that influence the course of the disease (prognostic factors). They may also look for factors that distinguish people who have a particular disease from those who do not have it (diagnostic factors).

The factors that influence the...