Harvesting is typically positively size-selective, targeting large individuals. This is expected to lead to reduced average body size and earlier maturation, i.e., faster life histories. Such changes can also affect traits seemingly unrelated to harvesting, including immunocompetence. The pace-of-life syndrome predicts that faster life histories are correlated with decreased immunocompetence, i.e., a negative association between positively size-selective harvesting and immunocompetence. However, the energetic trade-off between early growth and immunocompetence suggests the opposite pattern. Here, we empirically evaluated these predictions using an experimental system consisting of the ectoparasite Gyrodactylus turnbulli and lines of guppies Poecilia reticulata that had been subjected to either positively, randomly, or negatively size-selective harvest. We followed the infection progression of individually infected fish for 15 days. We found significant differences between the harvested lines: fish from the negative size-selection lines had the highest parasite loads. During the early phase of the infection, parasite loads were the lowest in the positive-harvested lines, whereas the terminal loads were the lowest for the randomly harvested lines. These results agree with the predictions from the energetic trade-off hypothesis but contradict with the pace-of-life syndrome ones. To our knowledge, this is the first demonstration of the consequences of size-selective harvesting on immunocompetence.