Net positive effects of early-life parasitism on animal host fitness

Early-life parasitism can negatively impact the growth, development, and survival of animal hosts. Parasitism can also positively influence developmental processes that increase host resistance, fecundity, and survival. Established processes include the development of resistance barriers, immune priming and training, acquired resistance, and behavior. However, few studies have calculated the overall, net effect of early-life parasitism on host fitness, and instead have focused on a particular life stage or just the cause (e.g., immune response) or consequence (e.g., survival). Indeed, several challenges prevent progress, including immune response specificity, dose-dependent immune responses, logistical feasibility, and analyzing imperfect datasets. Going forward, an integrative approach is needed to address the roadblocks that the field is currently facing.