The immune system has evolved, and continues to evolve, in response to the selection pressure that infections exert on animals in their natural environments, yet much of our understanding about how the immune system functions comes from studies of model species maintained in the almost complete absence of such environmental selection. The scientific discipline of immunology has among its aims the improvement of human and animal health by the application of immunological knowledge. As research on humans and domesticated animals is highly constrainedethically, logistically and financiallyexperimental animal models have become an invaluable tool for dissecting the functioning of the immune system. The house mouse (Mus musculus) is by far the most widely used animal model in immunological research but laboratory-reared mice provide a very narrow view of the immune systemthat of a well-fed and comfortably housed animal with minimal exposure to microbial pathogens. Indeed, so much of our immunological knowledge comes from studies of a very few highly inbred mouse strains thatto all intents and purposesour immunological knowledge is based on enormously detailed studies of very small numbers of individual mice. The limitations of studies in inbred strains of laboratory mice are well-recognized (Pedersen & Babayan 2011), but serious attempts to address these limitations have been few and far between. However, the emerging field of ecological immunology where free-living populations are studied in their natural habitat is beginning to redress this imbalance (Viney 2005; Martin 2006; Owen 2010; Abolins 2011). As demonstrated in the work by Boysen (2011) in this issue of Molecular Ecology, studies in wild animal populationsespecially free-living M.similar to musculusrepresent a valuable bridge between studies in humans and livestock and studies of captive animals.