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Insights into disease may be gained in a variety of different ways. Sometimes epidemiologists, those who study the occurrence of a disease, are able to show that a disease correlates with a specific food, poison, infectious organism, or parasite. A community physician may occasionally notice that he has seen more cases of a disease than might be expected. He might then report the existence of a cluster of cases in his community. Experience indicates that a cluster of a new syndrome is likely to prove significant, as have clusters of AIDS in the last decade or clusters of Legionnaires disease in the previous one. However, clusters of more common diseases observed in communities are likely to provide little information or to have little statistical validity. The authors discuss the statistical problems that make it difficult to determine if an observed cluster is actually related to some aspect of the disease or simply the result of random chance. For example, a physician in a community of 40,000 might suspect that more cases of amyotrophic lateral sclerosis have been seen over the last 10 years than would be expected in a community of this size; analysis of case records may confirm this hunch. To establish that this is a significant cluster of cases, other factors must be considered, such as the geographic stability of the community. If 20 percent of the people in this community move out each year and the same number move in, within approximately 10 years about 120,000 different people will pass through the physicians offices of the community. Three times as many cases of any disease are likely to be seen than would be expected in a population of 40,000. Other considerations may influence the appearance of clusters as well. It would be inappropriate to ignore all reports of disease clusters. Similarly, it would be impossible to follow-up all such reports with a detailed investigation. Careful data analysis is necessary to distinguish the community-based reports of disease clusters that are more likely to yield useful epidemiological insights. (Consumer Summary produced by Reliance Medical Information, Inc.)

Creutzfeldt-Jakob disease (CJD) is a brain disease which leads rapidly and inexorably to dementia, coma, vegetative state, and death. Some cases occur in family clusters, but the majority are sporadic. The disease may be transmitted by the tissues from CJD patients, and the infectious agent appears to be an unusual protein particle dubbed a 'prion,' rather than a conventional virus. The condition is found worldwide, and reports of its incidence range from 0.1 to 1.9 new cases per million people each year. In the 1970s, health officials in Israel observed that 40 percent of the new cases of CJD in that country were from Libya. This would suggest that Libyan Jews have an annual incidence rate of 31 per million, many times higher than anywhere else in the world. This finding directed the attention of health officials to this disease and toward Libyan Jews to determine if the initial finding is indeed true and what possible explanations might account for the observation. A survey was taken of all Jewish patients hospitalized with CJD between 1963 and 1987. In contrast with some illnesses, patients with CJD are invariably hospitalized towards the end of their lives, and thus such a survey is likely to include virtually all cases nationwide. A total of 114 cases were identified in this way. Forty-nine of these patients were born in Libya. The annual incidence rate was found to be 0.9 per million for most Israelis and 43 per million for Libyan Jews. If there were some sort of unidentified exposure in Libya which caused the development of CJD at such a high rate, then immigrants who spent more time there before emigrating should be at higher risk. This was not found, however. Investigating the families of the patients, it was found that 12 Libyan families had more than one case of CJD, in contrast with only two such families that were not of Libyan origin. These observations suggest that the reason for the enormous incidence of Creutzfeldt-Jakob disease among Libyan Jews is, in fact, due to a concentration of families carrying some genetic factor causing or contributing to the disease. (Consumer Summary produced by Reliance Medical Information, Inc.)