The standard models introduced in Chapter 2 compartmentalize the population only in terms of infection status and history—classifying individuals as susceptible, infected, or recovered—and modeling the number of individuals in each compartment. As such, there is only one degree of subdivision within the population. In this chapter, we introduce a second degree, further dividing the population into classes with similar behavioral characteristics. These characteristics should be chosen such that all members of a class have comparable risk of both contracting and transmitting infection.

Two clear motivating examples dominate the literature of models dealing with risk:
Finally, we consider levels of heterogeneity within the infected class, discriminating between individuals due to the time since infection.