Example of the interaction of evolutionary dynamics of a pathogen in an individual with the treatment of infection. Top left: Phenotypes (=properties) of different pathogen variants depend on the drug treatment. In the example, when treated with treatment 1, certain variants (black dots) can continue to multiply sufficiently to maintain the pathogen population (they are resistant). All other variants disappear over time. When treated with treatment 2, it is other variants that can prevail (red dots). Top right: Due to evolutionary distance, it is not clear if a resistant variant actually emerges before the pathogen population collapses. Bottom left: Population dynamics of the pathogen in the infected individual after the start of a treatment. In this case, a resistant variant emerges by chance and becomes established from ≈ day 120. Bottom right: Skillful choice of therapy keeps the pathogen population small and the resistant variant cannot establish itself, because intermediary variants become eliminated before the resistant variant emerges.