The current estimates on the transmission of pandemic (H1N1) 2009 are that one infected person may typically infects one to two people [24] [25] [26] . The transmissibility, b, is the product of the susceptibility of the population, the infectivity of the disease, and the number of contacts an individual has in a day [27, 28] . The susceptibility of the population is set to one, as it is believed few people are immune to pandemic (H1N1) 2009, and the number of contacts an individual has per day is assumed to be 16 [29] . The infectivity is found by 1:8=(16( a v z h mzd )), so that R 0 = 1.8 in a completely susceptible population and the infectivity is .0141. So b~0:23 gives the transmission rate, the fraction of contacts per day that is sufficient for the transmission of pandemic (H1N1) 2009. The baseline population size N for the model is set at one million people and all are initially in the susceptible class S. The initial infected fraction, I/N, is set at 0.00001 so that when N = 1000000, I = 10. The model scales linearly so that the initial population size N and the initial number of infected individuals I are both scaled up or down by the same factor. We assume that individuals will start wearing masks after 100 people are infected, Figure 4 . Sensitivity to < unc < unc . The number of cumulative cases is very sensitive to the value of the uncontrolled effective reproduction number (< unc ). Higher values of < unc result in a larger number of cumulative cases. A large difference in the number of cases is seen when the < unc is equal to 1.83 and when < unc is equal to 1.7; for such a slight difference in < unc the difference in the number of cases is quite large. doi:10.1371/journal.pone.0009018.g004 (6) once there is enough number of cases in a community to convince people to start wearing masks. We analyzed the impact of masks when 10%, 25%, and 50% of the population wear them.