In this study, we investigate the reciprocal interactions between host-parasite evolutionary dynamics and community structure. We quantify the impact of parasite presence on community composition as well as the impact of community composition (presence of alternative hosts) on virulence and prevalence of parasites. We use Daphnia and its microparasites as a model system, and carry out large-scale mesocosm experiments to quantify the interactions and dynamics under controled yet semi-natural conditions.