Without screening, the daily probability that an epidemic is initiated (p d ) increases, and becomes near certain once the number of infected travelers arriving undetected exceeds about 10 ( Figure 3B, solid line) . With screening and individual-based removal of infected individuals, p d follows a similar pattern only reduced somewhat. With screening in combination with flightbased quarantining, this probability is changed dramatically. After an initial rise it dips, to become essentially zero during the height of the epidemic in the source region ( Figure 3B , dotted line). This arises because once a flight has several infected travelers, the probability that at least one is detected approaches one (even if screening is imperfect), and all passengers on such a flight are quarantined. Once the epidemic starts to wane in the source region (assuming the unlikely event of the pandemic strain is restricted to the source region), the probability of initiation rises once again. The corresponding distribution of D 1 , the delay until The effects of R and the time from symptom onset to presentation on the probability that an infected traveler, having entered the wider community following arrival, will initiate an epidemic. There is no screening. doi:10.1371/journal.pone.0000143.g002 the epidemic is first initiated in the at-risk country, is bi-modal in the presence of screening ( Figure 3C) .