The delay contains a fairly substantial natural component, primarily due to the time it takes to increase the number of infectives in the source region sufficiently to make the chance of a recently infected traveler appreciable ( Figure 3A ), and the time (D 2 ) it takes for a local epidemic in the at-risk country to gather momentum following successful seeding ( Figure 4A ). In the absence of any interventions, the number of infected individuals who successfully enter the community of the at-risk country initially increases exponentially ( Figure 3A ). With individual-based removal of infected travelers, the number of individuals entering the at-risk country undetected by screening is proportionately reduced over the course of the epidemic ( Figure 3A ). With flightbased quarantining, the number of infected individuals entering the at-risk country undetected is dramatically reduced over the course of the epidemic, even for relatively insensitive screening ( Figure 3A ). With flight-based quarantining, the number of infected passengers slipping through undetected is bimodal, with the first peak occurring when the number of infected travelers attempting to travel is still in single figures.