The effects of heterogeneity in host susceptibility.

<p>We assume a host population that is composed of 1, 2, 4 or 6 classes of individuals, varying in their susceptibility to a virus. The number of classes is given in the legend, and applies to all three panels. (E.g. A value of ‘1’ indicates the infection probabilities are the same for all hosts, so in this instance the host population is homogeneous. ‘6’ indicates that there are six host classes with different susceptibilities to the virus) The following infection probabilities were assumed: 1 class, 10^−4.5; 2 classes, 10⁻⁴, 10⁻⁵; 4 classes, 10⁻³, 10⁻⁴; 10⁻⁵, 10⁻⁶; 6 classes, 10⁻², 10⁻³, 10⁻⁴; 10⁻⁵, 10⁻⁶, 10⁻⁷. The geometric mean infection probability is 10^−4.5 in all four cases. A virus population composed of two genotypes in a 1∶1 mixture, and no differences in infection probability for these genotypes, was assumed. In Panel A, the dose response relationship is illustrated. On the x-axis is the log of dose, and on the y-axis mortality. Note that as more host classes are introduced, the dose response relationship becomes shallower. In Panel B is the frequency of mixed-genotype infection, which follows a similar trend with dose. Panel C is the relationship between host mortality (x-axis) and the frequency of mixed-genotype infection (y-axis). The solid line is a 1∶1 relationship between mortality and the frequency of mixed-genotype infection. As a more heterogeneity is introduced in the host population, the frequency of mixed-genotype infection becomes higher and eventually approaches the 1∶1 line (both micro-parasite genotypes are established in all hosts at any level of mortality).