A disease cluster is an unusually high incidence of a particular disease or disorder occurring in close proximity in terms of both time and geography. Typically, when clusters are recognized, they are reported to public health departments in the local area. If clusters are of sufficient size and importance, they may be re-evaluated as outbreaks. The 1854 cholera outbreak which occurred in London is a classic example of such a cluster.
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Disease cluster Wikipedia(Text) CC BY-SA