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Epidemic disease outbreaks pose a significant risk to the stability and survival of many populations on earth. Current methods to understand how epidemic diseases transmit are often confounded by heterogeneity in infection rates amongst host populations. Endemic parasites, which are often less severe compared to epidemic diseases, may contribute to that variation by impacting on host biology and therefore altering the transmission of epidemic diseases. The effects of an endemic, gastrointestinal infection on host fitness and host interactions with an epidemic parasite were explored in a novel invertebrate system developed for this study. The chosen host was the German cockroach, Blattella germanica, which was infected with an endemic protozoan parasite, Gregarina blattarum alone or in co-infection with an entomopathogenic nematode, Steinernema carpocapsae, which causes epidemic outbreaks in host populations. There was evidence of density dependent regulation by the endemic parasite. Reductions in both host survival and fecundity during endemic infection both contributed to this regulation. The endemic parasite also had fitness costs for offspring from infected parents, who took longer to reach adulthood and were less likely to survive. Protozoan infected host populations had lower densities and showed less variance in population fluctuation compared to parasite free populations and the endemic parasite was generally found at high prevalence within the infected groups. When hosts infected with G. blattarum were exposed to S. carpocapsae, the resulting co-infection led to reduced host survival but also reduced emergence of nematode transmission stages. Hosts infected with G. blattarum also had differential immune responses to macro and microparasites which could alter host susceptibility to different types of infections. Infection with an endemic parasite caused substantial changes in the biology of the host, which may have important effects on host population ecology. Endemic parasites can also have considerable consequences for the transmission potential of more a virulent pathogen. These findings demonstrate the important of endemic infections which should be given greater consideration in future host-parasite studies.