One can define a gradient of infection G, which measures the tendency of the disease to either expand or shrink in a population with given configuration (defined by the number of individuals in each of the states S, I and R). For the SIS model, eradication of the disease is favored (G(i),0), irrespective of the fraction of infected, whenever R A 0 : l d Nw SI v1 (see Eq. 35 in Text S1), indicating how the presence of information (b H ,b I ) changes the basic reproductive ratio. This is illustrated in the upper panel of Figure 1 , which depicts G for different values of b I (assuming b H ƒb I ) and a fixed transmission probability l. The corresponding quasi-stationary distributions, which characterize the relative time the population spends in each configuration (and defined in Methods), are shown in the lower panel and clearly reflect the sign of G. Whenever G(i) is positive (negative), the dynamics will act to increase (decrease), on average, the number of infected. Figure 1 indicates how the availability of local yet reliable information hinders disease progression: For b I~0 :75 the interior root of G(i) disappears, making disease expansion unlikely in any configuration of the population.