Epidemiology: An introduction

2.2.3 Place: where do the problems occur?

The cholera epidemic of the 1850s, mentioned in Section 2, demonstrates the influence of geographical and environmental factors on the occurrence of disease. Disease patterns also vary internationally; for example, the global pandemic of HIV infection is not geographically uniformly distributed (United Nations, 1998), as Figure 9 shows.

Within countries, regional variations in the occurrence of diseases are not uncommon. In addition, differences in disease patterns between urban and rural communities are frequently observed. Even within one health district, mortality rates due to particular diseases may vary from one electoral ward to another.

Studies of immigrant populations focus on separate groups within geographical areas. One such population was that which migrated from Japan to the USA between 1890 and 1924. It was found that heart disease in Japan was only one-quarter the rate of that in the USA, whereas stroke and cerebral haemorrhage were two or three times more common (Marmot et al., 1975). Stomach cancer was five times more common in Japan, but cancer of the breast and prostate were very uncommon; cancer of the cervix was twice as common in Japan as in the USA. As the Japanese population settled in the USA, some of their disease patterns changed, and conditions such as stroke, cerebral haemorrhage and cancer of the cervix approached the rates of the community to which they had migrated. This suggests that environmental factors played a considerable part in the causation of these particular diseases. Cancer of the stomach also declined, but not to a very great extent (Hirayama et al., 1980). Cancer of the breast, however, did not alter significantly when the population had settled in the USA. Caution must be exercised when making international comparisons of illhealth. In developing countries, access to biomedical treatment and the availability of facilities for investigation can be restricted. This means that there may be problems with the accuracy and completeness of diagnosis. The age structure of the population in developing countries, with a greater proportion of young people than in the industrialised world, also makes comparisons of disease frequency difficult.

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