The 'next-generation operator' approach [17] is used to find an expression for the effective reproduction number (controlled) < con for our epidemic model when masks are used as an intervention strategy. The computation is done by linearizing the system of equations (2) around the disease-free equilibrium (DFE). The DFE has E, E m , I, and I m equal to zero with S 0 , S 0 m , and R 0 positive. Since there is no immunity from previous infection or vaccination R 0 is also equal to zero. The resulting fourdimensional linearized system is of the form dX dt~( The effective reproduction number < con is the largest eigenvalue of the matrix FV {1 [17] . Hence < con is the only non-zero eigenvalue of the matrix FV {1 and is given by the expression: Figure 2 with respirators is also seen here: as the masks effectiveness is higher the number of cumulative cases decreases and the number of cases also decreases if a higher percentage of people wear masks. However, the difference in the number of cumulative cases is not nearly as large when surgical masks are worn; this is due to their lower effectiveness. doi:10.1371/journal.pone.0009018.g003