Before revision:  The standard "compartment" models of the spread of disease through a population follow the law of mass action; that is, the rate of conversion of individuals from one compartment to another is directly proportional to the number of individuals in the former.
After revision:  The standard "compartment" models of the spread of disease through a population using compartmentalized differential equations, but do not incorporate population-level heterogeneity in disease susceptibility. We show that variation leads to the natural emergence of a power law in the force of infection (\beta I S^p), where the order p is a simple function of the rate of conversion of individuals from one compartment to another is directly proportional to the number of individuals in the former.