Unsampled host species may associate with parasites of interest, but it can be challenging to predict with which species a given parasite will associate. We tested the predictability of a parasite’s host range (the set of host species it can infect) using a large database of helminth parasites of fish and boosted regression trees (recommended R resource). Tad Dallas led the research, which was recently published in Parasitology. While host traits and environmental variables were predictive, the single best class of predictors was the parasite community itself.