A disease cluster is an unusually large aggregation of a relatively uncommon disease (medical condition) or event within a particular geographical location or period.^[1] Recognition of a cluster depends on its size being greater than would be expected by chance.^[1] Identification of a suspected disease cluster may initially depend on anecdotal evidence.^[1] Epidemiologists and biostatisticians then assess whether the suspected cluster corresponds to an actual increase of disease in the area.^[1] Typically, when clusters are recognized, they are reported to public health departments in the local area.^[2] If clusters are of sufficient size and importance, they may be re-evaluated as outbreaks.^{[citation needed]}

John Snow's pioneering investigation of the 1854 cholera outbreak in Soho, London, is seen as a classic example of the study of such a cluster.^{[citation needed]}