We model host evolution of costly resistance to infection and its dependence on environmental factors, such as nutrients. We find that higher nutrients can increase infection prevalence AND select for lower resistance. In turn, the model predicts that lower resistance drives infection prevalence even higher while depressing host density. The attached code performs the model analysis, produces the published figures, and conducts statistical analysis on the data (described below). We conducted a mesocosm experiment with mixtures of zooplankton host (Daphnia dentifera) genotypes, algal resources (Ankistrodesmus falcatus), and fungal parasites (Metschnikowia bicuspidata). Mesocosm populations were supplied with low or high nutrients (5 or 50 ug/L phosphorus and 100 or 1000 ug/L nitrogen). We measured densities of hosts along with age class (juvenile or adult), sex, infections status, and egg number and chlorophyll densities; these data are a subset of data published previously Walsman et al. Functional Ecology (https://doi.org/10.1111/1365-2435.14030; data at https://doi.org/10.5061/dryad.mw6m905zg). For the first time, we also report genotype frequencies for the mixed genotype treatments. Importantly, we found that high nutrients increased infection prevalence as well as selecting for the host genotype less resistant to infection; the resulting host evolution increased infection prevalence further and depressed host density. These data and code may be reused with citation of the corresponding publication ("‘Resistance is futile’: Weaker selection for resistance by abundant parasites increases prevalence and depresses host density" in The American Naturalist).