EPIDEMIOLOGY OF DISEASE
Epidemiology is 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.
Epidemiological 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.
The decreased 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.
Socioeconomic 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.
Some diseases 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 diseases.