Parasites represent as many as half of all species of organisms on Earth. This rich diversity is driven, in part, by host specificity. Most parasites are host specific, and some parasites are so specific that they only infest only one host species. Intriguingly, however, host specificity is in constant flux. Parasites can, and do, switch to new host species. In fact, one study estimates that as many as 61% of the parasites/pathogens affecting humans have zoonotic origins. Furthermore, human introductions of plants and animals into new geographic regions have exposed wildlife to new parasites/pathogens that may ultimately threaten the conservation of global biodiversity. My research focuses on the evolutionary ecology of host-parasite interactions. In particular, I am interested in using macroevolutionary information for hosts and their co-evolving parasites to generate testable hypotheses about ecological factors determining host-specificity.