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 strongly influences the rate of infection, while the infection process simultaneously sculpts the susceptibility distribution. These joint dynamics influence 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 .