Common-source outbreak

A common-source outbreak affects people exposed to the same source of an infectious agent or toxin. A brief exposure gives a point-source pattern; a continuing or intermittent source can produce a broader or repeated pattern. This classifies the exposure mechanism, whereas a propagated outbreak involves transmission between successive hosts.^[ref]

In an illustrative point-exposure model, everyone is exposed at time zero but the incubation period varies between people. Symptom onsets spread over subsequent days even though exposure was simultaneous. If the incubation-time probability density function is $f(u)$ and $M$ exposed people eventually develop symptoms, the expected onset count per unit time is $Mf(t)$, assuming complete observation. The width of the epidemic curve then reflects individual delays rather than the duration of exposure.

An investigator therefore combines an epidemic curve with individual exposure histories and comparison groups. A single peak does not by itself establish a shared source, and a shared source can be followed by secondary propagated outbreaks.