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| Block 1: Epidemiology |
| EPI1.1: WHAT IS EPIDEMIOLOGY? AN INTRODUCTION |
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| Block 1: Epidemiology |
| EPI1.1: WHAT IS EPIDEMIOLOGY? AN INTRODUCTION |
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In the epidemiology sessions generally a slightly different structure and content will apply.
As far as possible there will be a set of objectives at the beginning of the session which you need to be comfortable with by the end of that session. This will be followed by some learning activities as inputs somewhat akin to a lecture but with some worked examples. Additionally the input materials will be layered and you will be told what is introductory material, what is more advanced and what is required or optional. Some references will be given but generally speaking the materials you are receiving with this CDRom are sufficient for your purposes for the epidemiology sessions. Lastly, there will be in some, but not all sessions, a self test for you to do. When this is present it will come at the end of the session. You may redo this as many times as you wish until such time as you feel comfortable that you have mastered the concepts tested. You may also have to take formal epidemiology tests and assignments throughout the duration of the course, and these materials will be of great help to you and can be consulted frequently.
SESSION OBJECTIVE |
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At the end of this session you should have have some knowledge of the history and disciplinary development of epidemiology. |
The study of the distribution and determinants of health status and its determinants (including health services) in populations or groups as opposed to individuals with a view to promoting health and preventing and controlling disease (including improving health care effectiveness and efficiency).
Epidemiology is the science of occurrence of diseases in human populations. Disease occurrence is measured and related to different characteristics of individuals or their environments. (The word epidemiology consists of the Greek words epi = upon, demos = the people, and logos = doctrine, and thus means the study of disease in groups. Investigation of disease occurrence is not a new phenomenon. The development of epidemiologic theory and methods in recent decades, however, has opened new possibilities and stimulated interest within many fields of application.
For a long time the predominant interest in epidemiology was the area of infectious diseases. The occurrence of highly contagious infectious diseases varied in obvious ways, a nd often increased dramatically during so-called epidemics. It was found that individuals who had been in contact with sick people often became ill themselves and that those who recovered seldom got sick again. Such epidemiologic observations became the basis of theories about infectiousness and immunity and suggested effective means to prevent diseases, even before microorganisms and antibodies were discovered. One well known example is the classic study on cholera in London that was conducted by John Snow in 1854 (Snow 1855).
Early epidemiologic observations were not limited to infectious diseases; other diseases also displayed variation in their occurrence. The distribution of different malnutritive diseases was studied early in this century and related to certain characteristics of food composition. Even before essential nutrients, such as certain vitamins, had been identified, theories about the causes of malnutritive diseases were formulated, preventive means undertaken, and sick people successfully treated (eg scurvy and Vitamin C). Studies on the distribution of pellagra undertaken by Goldberger between 1915 and 1926 (Terns 1964) form a good example of this process.
During the last few decades increasing attention has focused on the epidemiology of malignant diseases. Epidemiologic studies contributed decisively to understanding the role of cigarette smoking in the occurrence of lung cancer. Other studies have shown that there is an association between exposure to some types of ionizing radiation and certain forms of cancer. Many epidemiologic studies have demonstrated the connection between exposure to certain chemical substances and some kinds of malignant tumors. Although knowledge about the etiologic mechanisms for these diseases is still rudimentary, epidemiologic investigations occasionally have provided a sufficient lead for the implementation of preventive measures.
Another current field of application of great importance is cardiovascular disease. In this century myocardial infarction has become a leading cause of death in the industrialized world. One plausible explanation for this increase is the profound change in what has come to be known as "lifestyle". The role of various components of lifestyle is not clear, however. We still lack fundamental knowledge on the relative importance of factors such as stress, limited physical activity, smoking, high intake of calories and high proportion of saturated fats, and we do not know what the relation is between these characteristics and elevated blood pressure, serum cholesterol and triglycerides (blood fats). In recent years, a large number of epidemiologic studies have evaluated the role of these and other characteristics in causing myocardial infarction to clarify ways in which the disease can be prevented. With similar questions in mind, other vascular diseases such as stroke have also been studied. In the past, attention has been focused mainlv on diseases with short duration (for example, acute infectious diseases), while in more recent years the focus has been increasingly on chronic diseases. This is now swinging back to infectious disease as increasing threats of conditions like HIV/AIDS and avian flu strains appear.
Chronic diseases are of great importance because they represent longstanding suffering for many people and a considerable burden to the healthcare system. An example is joint disease, such as rheumatoid arthritis. Even after corrections for differences in the age and sex distribution, substantial differences remain in the frequency of rheumatoid arthritis between different populations. On the other hand osteo-arthritis of the hip joints which has been taught for decades as "idiopathic" or having no known cause has been conclusively demonstrated to be linked to heavy manual work or certain sport exposures. Epidemiologists are now asking, "Which characteristics among the individuals (for example, genetic) or their environment (For example, exposure to infectious agents) explain these differences in morbidity from rheumatoid arthritis?" Another example is intestinal diseases such as ulcerative colitis and Crohn's disease. What affects the occurrence of these diseases and the risk of complications (For example, colon cancer)? Other epidemiologic studies have focused on variations in the frequency of birth defects and the impact of factors such as smoking, alcohol consumption, drug use, and infections during pregnancy.
Sometimes the starting point for an epidemiologic study is a certain characteristic or exposure rather than a disease. In studies of occupational hazards the starting point often is a characteristic of the occupational environment or work place; the effect of the exposure can be evaluated by measuring the health status or the frequency of disease occurrence in the occupational group and comparing it with a suitable reference group. For instance, how does exposure to asbestos in certain occupations affect the occurrence of diseases such as asbestosis, mesothelioma, and lung cancer? What is the relation of irregular working hours or occupational stress to health? What are the health hazards associated with mercury pollution from industries or use of insecticides in agriculture? In studies of side effects of drugs the starting point again is a certain characteristic (drug exposure) rather than a disease.
These examples of fields of application suggest a close connection between epidemiology and preventive medicine. Prevention programs are rarely implemented for an entire population, although they can be, as for example in flouridating drinking water. In general prevention interventions may evaluated by comparing disease rates among those receiving the preventive program with rates among those who do not.
In recent years the value of information about disease distribution for planning the delivery of health care has become more apparent. In several studies disease occurrence has been related to healthcare need, demand, and supply. There is also an increasing interest in studying the effectiveness of the healthcare system and/or of different treatments.
The common basis for these different applications of epidemiology is the study of disease occurrence and its relation to various characteristics of individuals or their environment. This latter includes health service characteristics
