Epidemiology and public health
Public health, broadly speaking, refers to collective actions to improve population
health.1 Epidemiology, one of the tools for improving public health, is used in several
ways (Figures 1.3–1.6). Early studies in epidemiology were concerned with the causes
(etiology) of communicable diseases, and such work continues to be essential since
it can lead to the identification of preventive methods. In this sense, epidemiology is
a basic medical science with the goal of improving the health of populations, and
especially the health of the disadvantaged.
Causation of disease
Although some diseases are caused solely by genetic factors, most result from an
interaction between genetic and environmental factors. Diabetes, for example, has
both genetic and environmental components. We define environment broadly to
include any biological, chemical, physical, psychological, economic or cultural factors
that can affect health (see Chapter 9). Personal behaviours
affect this interplay, and epidemiology is used to study groups
their influence and the effects of preventive interventions
Natural history of disease
Epidemiology is also concerned with the course and outcome
(natural history) of diseases in individuals and
Measuring disease frequency
Several measures of disease frequency are based on the concepts of prevalence and
incidence. Unfortunately, epidemiologists have not yet reached complete agreement
on the definitions of terms used in this field. In this text we generally use the terms
as defined in Last’s Dictionary of Epidemiology .11
Population at risk
An important factor in calculating measures of disease frequency is the correct estimate
of the numbers of people under study. Ideally these numbers should only include
people who are potentially susceptible to the diseases being studied. For instance,
men should not be included when calculating the frequency of cervical cancer
The people who are susceptible to a given disease are called the population at
risk, and can be defined by demographic, geographic or environmental factors. For
instance, occupational injuries occur only among working people, so the population
at risk is the workforce; in some countries brucellosis occurs only among people
handling infected animals, so the population at risk consists of those working on
farms and in slaughterhouses. Incidence and prevalence
The incidence of disease represents the rate of occurrence of new cases arising in a
given period in a specified population, while prevalence is the frequency of existing
cases in a defined population at a given point in time. These are fundamentally different
ways of measuring occurrence (see Table 2.2) and the relation between
incidence and prevalence varies among diseases. There may be low incidence and a
high prevalence – as for diabetes – or a high incidence and a low prevalence – as for
the common cold. Colds occur more frequen