Responding to reported clusters of common diseases: the case of multiple sclerosis.

Reports of disease clustering are becoming ever more common, and there is increasing pressure on public health agencies to respond rapidly and appropriately to these reports. We investigated a cluster of five cases of MS occurring in a small multidisciplinary hospital department. Data were collected by a variety of methods, including measurement and description of the workplace, completion of survey instruments by department staff, and construction of case histories of persons with MS. The results indicated that the department comprised a high-risk population and that only one case of MS could have any possible etiologic significance. Investigators should consider a number of factors when evaluating disease clusters, including the accuracy of diagnosis, the induction period and cause of the disease, and possible biases in the population at risk. Additionally, boundaries should not encircle the cases that led to identification of the cluster and should reflect environmental significance.