Figure 3. Functional behavior of IR with time. Although infectivity of an asymptomatic person is constant with time (IR 0.25), infectivity of a symptomatic person changes from infectious presymptomatic (IR 0.25) to early infectious symptomatic (IR 1.0) to late symptomatic (IR 0.375). A symptomatic person with mean state periods as denoted in Figure 2 is shown in gray (asymptomatic with dashed line). Because state periods are different for each person (given by exponential distributions) and half of the infected persons are asymptomatic, the average population scale IR in time is smoothed as shown in blue. Both disease state periods and IR values were chosen to honor the clinically derived natural history of influenza (12–14), selected viral shedding data shown as open red squares (15), and the model of Ferguson et al. (10,11).