Outbreaks generally involve infectious disease. The problem is unexpected; an immediate response may be demanded; public health epidemiologists must travel to and work in the field to solve the problem; and the extent of the investigation is likely to be limited because of the need for timely intervention.

A cluster usually refers to a uncommon diseases of noninfectious origin (e.g., leukemia, spontaneous abortions, suicides) which are often perceived to be due to environmental exposures. Cluster studies are sometimes called �small area analyses� because they typically are carried out at a subregional level at which routine reporting of disease rates is unlikely.