Schematic representation of a host population models that includes the possibility of resistance loss.

<p>A modified implementation of a previous host population model <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0080775#pone.0080775-Bonhoeffer1" target="_blank">[40]</a> under a combination of two drugs <i>a</i> and <i>b</i> (Equation S6 and S7 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0080775#pone.0080775.s005" target="_blank">File S1</a>) takes into account the possibility of resistance loss. Hosts can be infected by pathogens of four different types: wild type, a-resistant, b-resistant and a,b-resistant. The numbers of individuals infected are correspondingly represented by variables <i>y_w</i>, <i>y_a</i>, <i>y_b</i>, and <i>y_a,b</i>. The original model <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0080775#pone.0080775-Bonhoeffer1" target="_blank">[40]</a> considered only the possibility of acquiring resistance (black arrows). In our modified host population model, motivated by our findings in the single host model, we assume that a nonzero resistance-decaying rate can cause loss of resistance (red arrows).