Of course, the likelihood of a drug‐adapted strategy becoming fixed in the parasite population depends on the frequency that parasites encounter drug‐treated hosts, the benefits of altered patterns of conversion in a drug‐treated host, as well as the costs of that strategy in an untreated host. Using the fitness estimates for the different strategies in different environments (Table B.1 in Appendix B), we calculate the expected fitness for the drug‐adapted and non‐drug‐adapted strategies in a host population where some proportion of hosts are treated (Figure B.1). If b is the increase in fitness achieved by the drug‐adapted strain in the presence of drugs (i.e., the benefit), c is the reduced fitness of the drug‐adapted strain in an untreated host (i.e., the cost), and f is the proportion of infected hosts that are drug‐treated, then it is trivial to show (see Appendix B) that the drug‐adapted strategy has a higher fitness than the non‐drug‐adapted strategy when(12)f>cc+b.