Dynamics in Space: The herd immunity threshold and the critical community size are key dynamical concepts in understanding the local persistence of infectious disease.  The former defines the deterministic conditions necessary for an infection to spread and the latter is describes the phenomenological observation that the rate at which local incidence drops to 0 (“fade-out”), due to failure of infected individuals to successfully transmit, decreases as communities increase in size. Despite conditions that preclude persistent transmission (i.e. R0<1) or increase the likelihood of local stochastic fade-out, infection may persist at the regional scale through metapopulation dynamics. A metapopulation, as originally defined Hanski 2008, is a collection of sub-populations, or patches, that each have a non-zero probability of extinction, but are maintained through a process of continual re-establishment due to migration from extant to extinct patches.  In the context of infectious disease, the patches of a metapopulation are consistent with small outbreaks in local communities that are likely fade-out locally, either deterministically because R0<1 or stochastically because the communities are small Grenfell 1997.  Consistent with the theory of metapopulations, infection may be maintained regionally if there is sufficient migration between patches that each patch is likely to seed infection in another patch prior to local fade-out.  Thus, this dynamic phenomenon creates regional stability (i.e. persistence) despite local instability (i.e. frequent and predictable fade-out).