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  Epidemiology (Greek epidemia, ‘prevalence of disease’ + ology), in the life sciences, is the study of factors which determine the behaviour of a disease within a community. The epidemiologist is thus interested in the complex interactions between the host and parasite populations (see parasitism). Statistical techniques are used to determine factors such as frequency of infection, efficiency of transmission and susceptibility of subpopulations and to assess the significance of a wide range of associated factors such as dietary habits, social position, seasonal influences, etc. In a famous example, the discovery of infectious hepatitis was worked out by a Yorkshire family doctor who collated the information about the cases in his village and linked them all with a children\'s tea party. In another famous study doctors\' life histories were logged and followed, and it emerged that smoking cigarettes and lung cancer were linked. In the first example subsequent studies revealed a viral cause for the disease; in the second subsequent studies have strengthened the predictive value of the observation of the link between smoking and lung cancer although the causal agent has never been isolated.

Such data, collected and collated worldwide, builds up a picture of the relationships between causes of disease and behaviour-patterns, and enables the design of strategies to deal with disease. The World Health Organization uses such global epidemiological information to plan programmes of vaccination and health education of the type which led, in this century, to the eradication of smallpox.

In field trials, epidemiology could be used to test (for example) if vaccination alters the incidence of a disease, or to compare vaccination with a different intervention, like ensuring a supply of purified water. Epidemiology could also be used to test the effectiveness of a treatment, for example, the use of a specific drug to treat a particular illness. The process is to choose two comparable groups of patients of adequate size, to give the drug to one group and an indistinguishable preparation (the placebo) to the other group. In due course the results are tallied and if the test group fared better than the control group, then the treatment has been effective. (However, it has been observed that if the operators of the trial know the content of the preparation being dispensed they are able to influence the outcome of the trial, by unwittingly showing enthusiasm, compassion or over-inquisitiveness. Thus was devised the double-blind controlled trial in which neither prescriber nor recipient knew what was in the medicine. Not many medical or other treatments have been tested in this way, and even after trials have been concluded and published, uses of treatments are not necessarily altered.)

Epidemiological studies can be used to test the utility and effectiveness of any treatment or intervention: that is to say, it is not necessary to know how a given treatment works, but it is possible to design a study to find out if it works well enough to be worthwhile doing. Epidemiological studies can also be used to test the validity of information about factors causing or influencing disease, thereby suggesting potentially useful paths to investigate. RB TG

See also aetiology; medicine; parasitology; toxicology; virology.Further reading Craig Jones, Anthropology and Epidemiology.



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