Epidemic curves are an almost universal element of outbreak investigation. They may provide a valuable guide at the outset to the nature of the source of the outbreak. They may help characterise the outbreak as arising from a point source (a discrete confined event in a particular place and time), a continuous common source, person to person transmission or a combination of these. This information will usually provide early guidance for both generating hypothesises that may be tested during the investigation, and informing early provisional measures to control the outbreak. They are also a useful adjunct to monitoring effectiveness of control measures put in place during the outbreak.
Epidemic curves should be almost universally constructed at the start of the investigation, (They are in practise so potentially useful and available that not preparing them at the outset, with whatever information about cases is to hand should be positively justified).
The epidemic curve is a representation of what is known about the occurence of the cases in time. Usually it is constructed simply as a histogram of the onset date of cases over time. This apparently simple prescription however requires the application of a consistent definition for cases thought to be part of the outbreak, and initial case details may be fragmentary and incomplete. Epidemic curves will usually be successively developed with more precise case definitions and updated as the outbreak investigation proceeds.
Conceptually epidemic curves can also be constructed in space as well as and time. However the geographic information systems (GIS) to support this may not always be immediately available, and often the geographic information is a useful complement to information available from time distribution alone, rather than diagnostic in itself.