Outliers may be informative
An early case may be unrelated to the outbreak, may be the source of the epidemic (such as an infected food handler) or a person who was exposed early. Late cases may also be unrelated, but they may also be secondary cases or persons exposed later than most.
Tips on Interpreting Epidemic Curves
1) Consider the overall shape of the epidemic curve. The shape will be determined by the incubation period and whether it is a common source versus propagated epidemic.
2) Find the peak of the outbreak and count back on the time axis one incubation period. Also identify the earliest case in the outbreak and count back the minimum incubation period. If these dates are at all close, they may identify the time period during which the cases became exposed.
3) In a point source epidemic with a known disease, you can use the epidemic curve to calculate a likely period of exposure. This provides two advantages:
a) You can identify the likely period of exposure, and this will help with directing questions to the cases about possible exposures.
b) It may point to the index case.
4) If the causative organism has not yet been identified, the epidemic curve may help by enabling you to calculate the incubation period.