Insect nucleopolyhedroviruses (NPVs) are among the most important pathogens that regulate insect populations. Although these pathogens have been the subject of a great deal of research, many aspects of their transmission remain poorly understood. The transmission of NPV is controlled by a complex set of interactions between host, virus, and the environment. I investigated the effects of three different factors on the transmission of LdNPV in the gypsy moth, (Lymantria dispar), using laboratory and field experiments, DNA analysis, and mathematical modeling. The three effects examined were: 1) the effect of density-dependent disease resistance, 2) the effect of vectoring by avian predators, and 3) the potential occurrence of vertical transmission of the virus. My main results are as follows: 1) The resistance of gypsy moth larvae to NPV decreased as larval density increased, perhaps due to the stress associated with high densities, and this relationship is predicted to have a stabilizing effect on population dynamics. 2) In aviary cage experiments, bird species showed differential effectiveness in spreading the virus from infected to healthy larvae on a local scale. This difference was due more to differences in prey processing behaviors among the birds than to differences in the amount of NPV or speed at which NPV passed through the bird guts. The highest level of transmission was achieved by red-eyed vireos, largely because of their strategy of beating larvae against a branch before ingestion, resulting in rupture of the larval cuticle and scattering of viral occlusion bodies. 3) We found no evidence for vertical transmission of lethal NPV infection in the gypsy moth. Although low levels of virus were detected in the tissues of adult female moths by real-time PCR, there was no organ that showed consistently positive levels of NPV. Furthermore, the putative virus from female tissues was not infectious to larvae in our studies, and therefore may represent either latent virus or non-occluded virions. Together, these results contribute significantly to our understanding of the processes governing NPV transmission in gypsy moths and are directly relevant to the host-pathogen dynamics of many other systems.