3.1 Introduction
The goals of this chapter are to provide an introduction to three different methods for formulating stochastic epidemic models that relate directly to their deterministic counterparts, to illustrate some of the techniques for analyzing them, and to show the similarities between the three methods. Three types of stochastic modeling processes are described: (1) a discrete time Markov chain (DTMC) model, (2) a continuous time Markov chain (CTMC) model, and (3) a stochastic differential equation (SDE) model. These stochastic processes differ in the underlying assumptions regarding the time and the state variables. In a DTMC model, the time and the state variables are discrete. In a CTMC model, time is continuous, but the state variable is discrete. Finally, the SDE model is based on a diffusion process, where both the time and the state variables are continuous.