the study of how diseases spread in populations in relation to their causal
factors. Consequently, epidemiology is largely concerned with the collection
and interpretation of data about diseases in groups of people rather than in
individuals. The types of data collected in epidemiological studies provide
information about the etiology of the diseases, whether there is a need for
screening or the introduction of other preventative measures and whether health
care facilities are appropriate.
The prevalence of a disease refers to the
proportion of people in a population affected at a specific time. The incidence rate is the number of new
cases of a disease in a population occurring within a specified period of time.
studies can often provide information about the cause(s) of diseases. Thus if a
disease has a high incidence in a particular region or population, then the
disease may have a genetic origin or it may be caused by environmental factors
peculiar to that area. Epidemiological studies of migrant populations are
especially useful since they can provide valuable information on the etiology
of a disease. A case in point might be where a migrant population has a high
incidence of a particular disease and then moves to another geographical area
where the incidence of the same disease is low. If the incidence of disease in
the migrant population remains high, then it is likely that the disease has a
genetic basis. If, however, the incidence in the migrant population decreases
to the level of the new geographical region, then environmental factors
probably play a role in its etiology.
The data on the
incidence of some diseases are very reliable. This is especially so for some infectious
diseases and cancers that arenotifiable.
Clinicians are legally required to supply details of all new cases of diseases
on the notifiable list to a central register. However, obtaining data on the
incidence of other diseases can be difficult. For most diseases, the data
obtained refer to mortality rates for that disease based on the causes listed
on death certificates. This method of obtaining data has the major limitation
of underestimating the incidence if the disease does not have a fatal outcome.
The incidence of certain diseases changes with time and also can vary considerably from one country to another and even within different regions of the same country.
These differences are particularly marked between developing and
developed countries. Infectious diseases and malnutrition are still more
prevalent in developing countries, while in the developed world, the incidence
of many infectious diseases has been reduced dramatically in the last 100
years. The infant mortality rate is often used as a measure of health related
to socioeconomic status. In general, the infant mortality rate is higher in
developing compared with developed countries.
incidence of many diseases in developed countries may reflect changes in
exposure to causative agents as well as the effects of preventative measures.
For example, the reduction of diseases such as cholera is associated, in a
large part, with improved public health measures. Improvements in sanitation,
sewage and hygiene have had a considerable impact in reducing the incidence of
many infectious diseases. Mass immunization against infectious diseases, such
as polio, has had enormous beneficial effects in reducing disease in the
population as a whole. Unfortunately the reduction in infectious diseases has
been accompanied by an increasing incidence of other diseases, such as
cardiovascular diseases, diabetes, several types of cancers and psychiatric
diseases. All are associated with aging and, to a certain extent, this may reflect
the increased life expectancy in the developed countries: people are not killed
by infectious diseases and live longer. Some evidence does suggest that the
increased incidence of these diseases is also due to changes in diet such as
increased consumption of saturated fats and other lifestyle factors, for
instance a lack of exercise. Intervention
studies aimed at changing diet and lifestyle factors in an attempt to
reduce the incidence of these diseases are already proving beneficial.
factors can also influence the incidence of many diseases. Poverty tends to be
associated with an increased incidence of malnutrition and malnourished
individuals are more susceptible to infectious diseases. Overcrowding is known
to promote the spread of infectious diseases resulting in epidemics.
have a high incidence in populations associated with certain occupations. For
example, coal workers have a high incidence of pneumoconiosis caused by
inhalation of coal dust and, in the past, workers with asbestos faced a high
risk of asbestosis, and of developing mesothelioma of the lung. Occupational
hazards need to be identified and minimized to reduce the incidence of these