How do you differentiate a point source from a propagating epidemic using the epidemic curve?

• A. Point Source: Epidemic curve skewed to the right; Propagating: skewed to the left
• B. Point Source: skewed to the left; Propagating: skewed to the right
• C. Point Source: straight line; Propagating: multi-peaked
• D. Point Source: vertical axis; Propagating: horizontal axis

Answer: (A)

The main idea is that the shape of the epidemic curve reflects how cases accumulate over time based on how the disease spreads. A point source outbreak comes from a single exposure (or a very short exposure window), so most people become ill within about one incubation period. This creates a rapid rise to a peak followed by a tail as later cases appear due to incubation differences, giving the curve a longer right-hand tail (skewed to the right).

In contrast, a propagating, person-to-person outbreak produces new cases in successive generations. Cases appear over several incubation periods with possible waves, so the curve is broader and can have multiple bumps. This pattern tends to place the bulk of cases later in time, producing a curve that is skewed toward the left (longer tail on the earlier side).

So the best pairing is: point source with a right-skewed curve, and propagating with a left-skewed curve. The other descriptions don’t match how single-exposure outbreaks cluster quickly in time or how ongoing transmission creates multiple peaks over an extended period.