GIS in health L. N. Balaji Chief, Strategic Planning, Monitoring and Evaluation, UNICEF, New Delhi lnbalaji@unicef.org The ability of GIS to "overlay" different entities based on their common geographic occurrence that makes it a very valuable tool in epidemiological research. The idea that place and location can influence health is a very old and familiar concept in Western Medicine. As far back as the time of Hippocrates, (460-370 BC), the father of Modern Medicine, physicians have observed that certain diseases seem to occur in some places and not in others. Hippocrates was constantly seeking the causes of disease. He studied such things as climate, water, clothing, diet, habits of eating and drinking and the effect they had in producing disease. The Hippocratic concept of health and disease stressed the relation between man and his environment. People have also been aware of the process of disease diffusion across geographic regions for centuries, even during times when the cause of the disease - aetiology - was a mystery. Frequently, attempts to understand why certain diseases seem to only occur in certain places and not others has led to new insights into the nature of the disease itself.  Fig. 1:Map of Cholera deaths in London, 1840s as done by John Snow A historic milestone in the evolution of medicine is the "great sanitary awakening" which took place in England in the mid-nineteenth century and gradually spread to other countries. In a classic study undertaken in 1854, John Snow (see Fig.1) demonstrated the utility of mapping disease outbreaks to gain insights as to their cause - or to coin a phrase, to "get a handle on a disease and off the water pump". Snow, an anaesthesiologist, mapped the distribution of cholera cases in Soho, London during an epidemic. He discovered that the highest density of cases occurred in households, which used the public pump on Broad Street as their water source. Epidemiology and Medical Geography As the study of epidemiology (epi=among; demos=people; logos=study) matured, simultaneously the discipline of Medical Geography (defined as the branch of Human Geography concerned with the geographic aspects of health (status) and health care (systems). Epidemiology, on the other hand, has been defined variously through times: as the branch of medical science which treats epidemics (Parkin, 1873); the study of disease, as a mass phenomenon (Greenwood, 1934); the study of the distribution and determinants of disease frequency in man (MacMohan, 1960) and now as the study of the distribution and determinants of health-related states and events in populations, and the application of this study to control health problems. Thus it can clearly be seen that both these disciplines have depended heavily on some form of mapping and understanding of the health, diseases and systems in different populations. Inherent in the definition of epidemiology is measurement of 'frequency', 'distribution', 'determinants' of disease. All of these require GIS application to make the information easy to understand, interpret and take action. Until relatively recently, doctors and public health professionals measured health strictly in terms of indicators of ill-health such as morbidity and mortality. There is however, a long tradition in the allied fields of Medicine viz. Epidemiology, Public Health and Medical Geography to examine the distribution of disease and death at various geographic scales, in an attempt to determine if the presence or absence of particular illness is associated with some factor(s) in the social or physical environment. In the case of infectious diseases, there is the added dimension of examining the diffusion of disease through space over a given period of time. Although mapping of disease can be relatively straightforward, interpreting spatially referenced disease data can sometimes be challenging. Regardless of the difficulties in data acquisition, map representation, scale, statistical analysis, and the interpretation and utility of results, the study of disease distribution may well be the most challenging and fascinating research area.
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