- Bumble bees (Hymenoptera: Apidae) are important native pollinators in wild and agricultural systems, and are one of the few groups of native bees commercially bred for use in the pollination of a range of crops. In recent years, declines in bumble bees have been reported globally. One factor implicated in these declines, believed to affect bumble bee colonies in the wild and during rearing, is natural enemies. A diversity of fungi, protozoa, nematodes, and parasitoids has been reported to affect bumble bees, to varying extents, in different parts of the world. In contrast to reports of decline elsewhere, bumble bees have been thriving in Oregon on the West Coast of the U.S.A.. In particular, the agriculturally rich Willamette Valley in the western part of the state appears to be fostering several species. Little is known, however, about the natural enemies of bumble bees in this region. The objectives of this thesis were to: (1) identify pathogens and parasites in (a) bumble bees from the wild, and (b) bumble bees reared in captivity and (2) examine the effects of disease on bee hosts. Bumble bee queens and workers were collected from diverse locations in the Willamette Valley, in spring and summer. Bombus mixtus, Bombus nevadensis, and Bombus vosnesenskii collected from the wild were dissected and examined for pathogens and parasites, and these organisms were identified using morphological and molecular characteristics. Queens of Bombus griseocollis, Bombus nevadensis, and Bombus vosnesenskii were reared in captivity, and those that died or did not initiate nests were also examined for pathogens and parasites. In addition, the immune responses of healthy and infected bees were compared, to examine the effects of a common bumble bee gut pathogen Crithidia bombi (Kinetoplastida: Trypanosomatidae) on its host. This thesis reports that wild bumble bees in western Oregon are infected with the protozoa Apicystis bombi (Neogregarinida: Ophrocystidae) and Crithidia bombi, the fungus/microsporidian Nosema bombi (Microsporidia: Nosematidae), the nematode Sphaerularia bombi (Nematoda: Tylenchida), and two dipteran parasitoids, one unknown and one belonging to the family Conopidae (Diptera: Conopidae). In addition, this research presents the first ever report of infection by the larval pathogen Ascosphaera apis (Ascomycota: Ascosphaerales), in reared bumble bee adults. New host and distribution records are also presented for C. bombi, N. bombi, and S. bombi. The study found that bumble bees infected with C. bombi had lower activated levels of immune responses than healthy bees. The thesis discusses the long-term implications of study findings, and addresses the threat that bumble bee diseases pose to bee pollinators through shared environments.