The epidemiological model used consisted of a metapopulation system with a discrete-time SEIR model (Eq. 1). For each city, S t is the number of susceptibles in the city at time t, E t is the number of infected but not yet infectious individuals, I t is the number of infectious individuals resident in the locality, N is the population residing in the locality (assumed constant throughout the simulation), and n t is the number of individuals visiting the locality, Θ t is the number of visitors who are infectious. The parameters used were taken from Lipsitch et al. (2003) [12] to represent a disease like SARS with an estimated basic reproduction number (R 0 ) of 2.2 to 3.6 ( Table 1) .