Evidence of multiscale, resurgent behavior in simulated data. (*A*–*C*) Histograms of epidemic size ψ for three configurations of the model. (*A*) The bimodal size distribution generated from 5,000 simulations of a homogeneously mixing population of size *N* = 102,400 and reproduction number *R*_{0} = 3. (*B* and *C*) The same size population as in *A* is hierarchically structured with branching ratio *b* = 4, depth *l* = 5, and group size *n* = 100, and for both of these examples, *P*_{0} = 0.35 and ξ = 0.35. The models for *B* and *C* differ in the reproductive number: *R*_{0} = 3 and 12, respectively. Nevertheless, both *B* and *C* show similar size distributions with modes near ψ = 0 and a relatively flat distribution for ψ > 0, qualitatively similar in form to that of the Icelandic data. (*D*–*G*) Example time series of total new cases, where in all cases, simulation parameters are *R*_{0} = 3 and, again, *N* = 102,400. For *D*, the population is homogeneously mixing, and for *E*–*G*, the population is structured according to our model with the same parameters as for *B* and *C*. For the random mixing case of *D*, a typical epidemic trajectory rises rapidly once and then declines to zero, infecting most of the population in the process (see gray sidebar). In the examples with population structure, by contrast, epidemic trajectories exhibit dramatic resurgence, endure for markedly different time intervals, and infect very different fractions of the population (see gray sidebars), depending solely on stochastic fluctuations (i.e., all parameters are held constant).

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