Fig 1. Illustration of the proposed mechanisms by which resistance can be lost from host populations over time, using snails and their trematode parasites.

A) Resistance can be lost if parasites counteradapt to overcome mechanisms of host resistance. B) As resistance spreads in a population, parasite prevalence will typically decrease (due to lack of suitable hosts), thus reducing the strength of parasite-mediated selection. If resistance carries a fitness cost, it should be selected against in this new environment, and susceptible host types will once again become common. C) If resistance is lost (either by mutation or phenotypic change) at a constant rate, the continual regeneration of susceptible host types in an otherwise resistant population can maintain a parasite population in an environment dominated by resistant host types.