Before revision:  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 distribution shape. p is significantly greater than one for reasonable variances, suggesting that conventional epidemic models make extreme assumptions about the absence of variance in susceptibility .
After revision:  We show that variation leads to the natural emergence of a power law in the force of infection and are, in turn, influenced by the shape of the initial variability. Intriguingly, we find that certain susceptibility distributions (the exponential and the gamma) are unchanged through the course of the outbreak, and lead naturally to power-law behavior in the force of infection; other distributions often tend towards these "eigen-distributions" through the process of contagion .