This review on the effects of opiate use on infectious diseases discusses the complete spectrum of infections in the opiate user, including those of the lung, the GI tract, the skin, the skeletal system, and the CNS. There is both increased prevalence and increased severity of bacterial and viral infections in injection drug users with the outcome of increased morbidity and mortality. The experimental administration of opiates has lead to a greater understanding of the effects on susceptibility to and progression of infectious diseases. Animal models of opiate dependence and infection are reviewed with specific attention to cases in which the opiate-mediated effects are harmful and in which cases they are beneficial.