Although research has focused on density-dependent responses to single parasite infections, much less in known about how parasite density affects the more common scenario in nature, co-infections. We investigate how parasite density alters co-infection dynamics by simultaneously exposing Cuban treefrogs (Osteopilus septentrionalis), in all pairwise combinations and at a range of doses, to: the nematode Aplectana hamatospicula, the chytrid fungus (Batrachochytrium dendrobatidis), and Ranavirus. The latter two parasites are implicated in widespread amphibian declines. We found that all pairwise co-infections were density dependent, but some were positively and others negatively density dependent, and these effects drove host pathology. Also, all co-infections were highly asymmetric – strong in one direction and weak in the other – consistent with weak and asymmetric interactions dominating food webs and mutualistic networks. These findings suggest that the null expectation for co-infections should be that they are density-dependent, asymmetric, and important to host health.