Over the last decade, there has been a major shift in the study of adaptive patterns and processes towards including the role of host-parasite interactions, informed by concepts from evolutionary ecology. As a consequence, a number of major questions have emerged. For example, how genetics affects host-parasite interactions, whether parasitism selects for offspring diversification, whether parasite virulence is an adaptive trait, and what constrains the use of the host's immune defences. Using bumblebees, Bombus spp, and their parasites as a model system, answers to some of these questions have been found, while at the same time the complexity of the interaction has led expectations away from simple theoretical models. In addition, the results have also led to the unexpected discovery of novel phenomena concerning, for instance, female mating strategies.