Figure A6. . Transmission probability distribution. The probability of transmission of respiratory pathogens depends on the amount of shedding, distance, duration of contact, and environmental factors such as temperature and humidity. Reports on SARS epidemiology suggest a bimodal distribution of transmission probabilities: close contacts in hospitals may have had high probabilities of transmission while typical contacts in schools, workplaces, and shopping malls may have had low probabilities of transmission (2,3). We assumed this distribution of per day transmission probabilities across the entire network in our analysis. The first peak corresponds to household contacts, while the second peak (with higher probability of transmission) corresponds to all contacts occurring in healthcare settings. Note that this distribution includes the per day probabilities of transmission for all possible contacts in the network, and thus has a smaller mean than the estimates reported for SARS in which the probability of transmission was estimated for particular settings (2,3).