I work in a lab now that focuses on directed evolution. We typically look at a huge, diverse pool of molecules and then select and “breed” them to get molecular properties we want.
This is something that can be used on the macro scale, too. In thinking about diseases, it’s important to consider not just a patient, but the population dynamics of the pathogen. Malaria, for instance, can be exacerbated or ameliorated by the preferential treatment given to severe cases. If sever cases are allowed to go untreated and the victim is allowed to be visited by mosquitoes, then the most severe forms become more likely.
Constant vigilance in removing severe cases from the population by something as simple as bed nets for every sick person can dilute the population of virulent forms of the disease. Tricks like these are undoubtedly going to be more and more important as diseases learn to avoid our chemical cures.
Here’s a neat article on the subject over at Nature