Reading the shape of an outbreak
An epidemic curve plots case counts by date of onset. It typically opens with one or a few index cases, climbs through an exponential rise as transmission accelerates, reaches a peak when new infections stop outpacing recoveries and depletion of susceptibles, then declines as the susceptible pool shrinks or controls take hold. The overall silhouette encodes the tempo of spread.
Plotting by onset date rather than report date matters, because reporting delays can blur or shift the true timing of the wave.
What the curve hints at about the source
A sharp single spike suggests a point source where many people were exposed at once, while a broad rolling curve suggests ongoing person to person transmission. The distance from the first case to the peak relates to the incubation period and the speed of spread.
This is general educational information, not medical advice. Interpreting outbreak data and choosing responses are tasks for qualified public health professionals.