Many epidemiological models are based on a compartmental, Sampling Importance Resampling (SIR) framework; the host population is partitioned into those that are susceptible, infected, or immune to a particular pathogen. These models assume that the rate at which new infections are acquired is proportional to the number of encounters between susceptible and infected individuals, and leads to an effective reproductive ratio that depends on a threshold density of susceptibles. Thus, the models depend not only on parameters intrinsic to the disease such as latent and infectious periods, but also on contacts between infectious and susceptible hosts. Historically, structuring the population at risk into compartments permits subpopulations of varying risks to be represented. Compartmental models of this kind implicitly assume that the host population is well mixed, such that the probability of infection is equal for all.