Parasites vary widely in the diversity of hosts they infect: some parasite species are specialists—infecting just a single host species, while others are generalists, capable of infecting many. Understanding the factors that drive parasite host-generalism is of basic biological interest, but also directly relevant to predicting disease emergence in new host species, identifying parasites that are likely to have unidentified additional hosts, and assessing transmission risk. Here, we use mathematical models to make predictions about how variation in host body size and environmental temperature affect the evolution of parasite host-generalism. We then explore these predictions using a newly updated database of over 20 000 fish–macroparasite associations.
Results/Conclusions Theory predicts that parasites are more likely to evolve a generalist strategy when hosts are large-bodied, when variation in host body size is large, and in cooler environments. Within the database we see some evidence supporting these predictions, but also highlight mismatches between theory and data. By combining these two approaches, we establish a theoretical basis for interpreting empirical data on parasites’ host specificity and ident- ify key areas for future work that will help untangle the drivers of parasite host-generalism.