The type of model described here, compartmental and deterministic, is one of the most widely adopted for investigating the transmission dynamics of infections in populations. As noted earlier, the results from these models are completely determined by their structure, the values specified for the parameters, and the initial values of the state variables. One drawback of this is that when an infection is first introduced into a population, when it is fading out, when it is very rare, or when the population is very small, the effects of chance can be an important determinant of whether the infection is able to continue to propagate itself through the population. In such cases a model incorporating random or stochastic elements into the transmission dynamics process can help to determine the distribution of possible outcomes (Bailey (1975) provides a useful introduction to this approach); in a sense, a deterministic model provides information about the mean of such a possible distribution of outcomes (they are sometimes called ''mean field'' models). In the limited space available it would not be possible to discuss adequately the full range of types of models used in infectious disease epidemiology including, among others, individual-based models and network models; for this purpose the reader is referred to the references quoted in the Resources section.
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