= 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.<Principles of Epidemiology in Public Health Practice by the Centers for Disease Control and Prevention>[Lesson 1, section 11, Epidemic Patterns: common-source, continuous and intermittent sources]{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>.
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