1. Emerging infectious diseases may decimate populations or become endemic, but such worst-case scenarios do not occur as frequently as might be expected, even for virulent parasites.
2. One explanation for this apparent paradox is that rapid evolution of host resistance may diminish or terminate epidemics. Theoretical and empirical studies have shown that evolution in host–parasite systems can dramatically alter the prevalence and intensity of infection.
3. The potential for rapid evolution to protect host populations from negative effects of virulent parasites depends on the type of parasite-driven evolution that occurs. In some host–parasite systems, evolution of increased host resistance can terminate epidemics. However, evolution resulting from parasite-mediated disruptive selection might actually allow a disease to persist in the host population. Epidemics may also be sustained through coevolution of the parasite.
4. The rate of evolution and subsequent disease dynamics will be affected by both the diversity of the host population and the community context in which the host–parasite interaction is embedded. Predators, competitors and food resources can all affect the rates of evolution of hosts and parasites and interact with evolution to determine the outcome of epidemics.
5. Freshwater organisms have played an important role in the studies of eco-evolutionary dynamics. Rapid evolution in response to parasitism has been demonstrated in multiple freshwater host species, which appears to have protected some of these populations from the virulent effects of infectious diseases.
6. Studies of emerging infectious diseases in freshwater ecosystems should consider the possibility of evolution of hosts and/or parasites on ecological timescales, since this phenomenon can profoundly affect disease dynamics.