Not all infected travelers entering the community initiate a 'major' epidemic, even when the reproduction number (R) exceeds one. Quite generally, the distribution of the size of an epidemic initiated by an infected arrival is bimodal, with distinct peaks corresponding to a major epidemic and a minor outbreak ( Figure 1E ). In the latter event the outbreak simply fades out by chance despite there being ample susceptibles in the population for ongoing trans- In (B), the step illustrates the probabilistic removal of travelers who have completed their incubation period. In (D), the distribution of time since infection in (C) will have shifted to the right by an amount equal to the flight duration, and cases incubated in-flight may be detected by symptomatic screening, as will those symptomatic cases that were not detected previously. Screening sensitivity for this illustration is 60% on both departure and arrival. (E) Upon entering the community undetected, an infected traveler may initiate a minor (inconsequential) or major epidemic, depending on the characteristics of the disease and public health policy. doi:10.1371/journal.pone.0000143.g001 mission [11] . The number of cases in an outbreak that fades out is typically very small compared to an epidemic.