Global loss of biodiversity and emerging infectious diseases represent current research priorities as they seriously threaten human and wildlife welfare. These risks also apply to bumblebees which provide effective pollination service to wild plants and crops. Using the well-established Bombus-Crithidia system, the present thesis addresses multifaceted aspects of complex host-parasite interactions in order to unravel the individual contribution of genetic, density-dependent and environmental impact at different levels of biological organisations. Host density and parasite prevalence were positively associated at the population level. However, the relative contribution of genetic diversity – with respect to reduced Crithidia bombi prevalence in B. terrestris – is much higher. At the community level, my results additionally confirm the decisive role of host species identity and local community composition. Therefore, species-specific differences (for example in the transmission potential) also need to be considered because communities of low diversity may be dominated by potential superspreaders which tends to apply to B. lapidarius.