AN INTRODUCTION TO EPIDEMIOLOGY

The first essential concepts that students of health and illness need to understand are disease and illness. To researchers and clinicians working with physical health problems, disease refers to biological problems within organisms. In contrast, illness refers to the social experience and consequences of living with a disease. Using these terms, we might say that a man who is infected with the poliomyelitis virus must adapt to physical disability caused by the disease called polio and to changes in how others view him because he has the illness called polio.

The study of the distribution of disease within a population is known as epidemiology. This chapter and the next focus more specifically on social epide- miology, or how social behaviors and factors (such as social class or use of tobacco) affect the distribution of disease within a population. For example, whereas biolo- gists might investigate whether heart disease is more common among those with high cholesterol levels, social epidemiologists might investigate whether it is more common among those with high incomes.

But what do we mean when we say a disease is “more common” among one group than another? Data on infection with the human immunodeficiency virus (HIV), the virus that in its later stages causes acquired immunodefi- ciency syndrome (AIDS), provide a useful example. (This book uses the term HIV/AIDS to refer to the full range of illness from the earliest stages of infection with HIV to full-blown AIDS.)

As of 2014 (the latest data available as of 2018), more than twice as many Bra- zilians have HIV/AIDS as do Botswanans. At first glance, this might suggest that HIV/AIDS is a much greater problem in Brazil than in Botswana. However, Brazil’s population is much larger than that of Botswana. To take this difference into account, epidemiologists typically look at the rate rather than the number of HIV/AIDS cases in a population. Rate refers to the proportion of a specified population that experiences a given circumstance. For example, to learn the rate of a HIV/AIDS in Brazil, we would divide the number of people in Brazil who have the disease into the total number of people in the country. In 2013, the rate of HIV/AIDS among adults was 3 per 1000 adults in Brazil compared with 234 per 1000 adults in Botswana.