To locate the probable exposure period from the epidemic curve of an apparent point source outbreak, what is counted back from the peak?

1. The method needs two figures for the disease: its average incubation period and its minimum incubation period. 2. From the peak, or equivalently the median case, count back one average incubation period and note that date. 3. From the earliest case, count back the minimum incubation period and note that date too. 4. The two dates should be similar, and together they mark the probable period of exposure. 5. The minimum period is counted back from the first case, not from the peak, which is the trap in the third option. 6. Because the technique is imprecise, the window is then widened by roughly 20% to 50% on either side before questioning begins. _Source: CDC Principles of Epidemiology in Public Health Practice, 3rd Edition, section Interpreting an Epidemic Curve_