- Direct anthropogenic stressors have caused drastic declines in wildlife populations over the past two centuries. In the face of these threats, spillover of infectious disease from domestic animals and livestock into wildlife, and novel interactions between parasites and pathogens within wildlife communities, have further suppressed already vulnerable populations. As management officials and conservationists fight to counteract these influences, a sound understanding of how pathogens and parasites interact to shape host health, and the level of disease threat posed from outside species, has become paramount to ensuring long term population viability. Here I use two model systems to examine these interactions. The first study system (Chapters 1, 2, and 3) examines the potential implications of an immunosuppressive lentivirus, feline immunodeficiency virus (FIV), for structuring host health, immunity, and coinfection dynamics in a population of 219 free-ranging African lions (Panthera leo) living in Kruger National Park, South Africa. Similar to HIV in humans, FIV has been linked to an AIDS-like syndrome in domestic cats characterized by decreased functional immunity; alterations to biochemical, histological, and serological markers; and increased susceptibility to other parasites and pathogens. While recent evidence suggests similar mechanisms may be at play in FIV-infected lion hosts, less is known about the health implications of FIV for this species or what role coinfections may play in structuring disease outcome. In Chapter 1 I set the stage for this investigation by expanding the available toolbox for lion health research and creating a set of normal reference intervals against which health metrics for free-ranging lions can be compared. Using metrics and tools established in Chapter 1, I then use Chapters 2 and 3 to show that FIV exhibits wide scale immunosuppressive and negative health effects within lion hosts, but that parasite and pathogen communities facilitated by the virus may be of equal or greater importance for determining both health outcome and susceptibility to other coinfections when compared to FIV alone. In the second study system, I examine disease prevalence and incidence in a common, highly adaptable species, the feral cat (Felis catus), to determine its potential as a disease reservoir for sympatrically dwelling human and animal populations. Using a healthy population of 129 feral cats presented at a local trap-neuter-release program in Portland, Oregon, I show that prevalence and incidence of viral and bacterial pathogens, as well as endo- and ectoparasites, is high among this untreated population. Together, findings of these respective studies show potential points for disease intervention on both the parasite and host level.