Avian haemosporidian infections (of the genera Haemoproteus, Plasmodium and Leucocytozoon) can regulate passerine populations. Thus, reduction in the number of avian haemosporidian infections in a population, for example in recently introduced hosts, may facilitate host establishment or spread (i.e. enemy release). Alternatively, colonizers could decrease competitive ability of native individuals in the novel range by increasing the prevalence of avian haemosporidians in that native passerine community (i.e. novel weapons). However, whether either or both of these phenomena will occur is difficult to predict because infection risk can be highly heterogeneous and dependent upon the interaction of biotic and abiotic factors at the microclimate level, especially because of the important role of vectors for these parasites. Here, we describe which factors best predicted avian haemosporidian prevalence in populations of house sparrows Passer domesticus introduced to Kenya. House sparrows inhabit an invasion gradient in Kenya; they were introduced via the eastern port city of Mombasa in ˜ 1950 and have since spread west-ward across the country. This range expansion gave us the opportunity to examine how parasite prevalence changes over small spatiotemporal scales and what role is played by environmental and individual traits. Among all individuals, body mass was the strongest predictor of infection, with larger house sparrows being more likely to be infected. At the population level, capture month, precipitation (higher prevalence with more rainfall), and population age (increasing prevalence with increasing time since introduction) were important risk factors. Overall, haemosporidian prevalence in Kenyan house sparrows appears to be more strongly associated with individual characteristics rather than with time since introduction as was predicted, though this does not necessarily rule out a role for enemy release or novel weapons in this system.