A disease cluster is an unusually large aggregation of a relatively uncommon medical condition or event within a particular geographical location or period. Recognition of a cluster depends on its size being seen as greater than would be expected by the play of chance. Identification of a suspected disease cluster may initially depend on anecdotal evidence. Epidemiologists and biostatisticians need to assess whether the suspected cluster corresponds to an actual increase of disease in the area. 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.