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This epidemic curve is from a cholera outbreak in London in 1854 that was investigated by Dr. John Snow. Here the source was a community water well. The incubation period for cholera is 1-3 days. Even though many residents fled when the outbreak began, the source of infection remained, and the epidemic continued beyond the 3 day incubation period. If the residents had not fled, there would have been a plateau instead of a peak.

This is a continuous common source epidemic in which exposure to the source is prolonged over an extended period of time and may occur over more than one incubation period. The down slope of the curve may be very sharp if the common source is removed or gradual if the outbreak is allowed to exhaust itself.

[Source: CDC website on interpreting an epidemic curve]