Ecosystems consist of pairwise relationships that determine the characteristics of communities. Host-pathogen relationships are unique in ecological communities since hosts can also be considered as an ecosystem in which commensal microorganisms, parasites and host tissues compete for resources. At all ecosystem levels, resource availability produces bottom-up effects that can determine the outcome of competitive interactions. Nutrients are key resources due to their ability to influence life-history. Theories such as nutritional geometry provide a framework for the investigation of nutrient interactions with life-history traits. Insect models are common due to their economic importance as pests as well as some similarities with human biology, hence much of what is known about nutrition and immunity comes from insects. Using the Egyptian cotton leafworm, Spodoptera littoralis (Lepidoptera: Noctuidae) and its natural pathogen Xenorhabdus nematophilus (Enterobacteriaceae) as a model system, this thesis investigates the influence of host nutrition on pathogen virulence and host resistance/tolerance. Findings confirmed the role of dietary protein intake on S. littoralis resistance against pathogens (Chapter 2) and indicated possible carbohydrate-use by the caterpillar to tolerate infection (Chapter 3). Dietary macronutrient intake altered the proportion of key nutrient groups in the haemolymph as well as concentrations of individual nutrients (Chapter 4). Both in vitro and in vivo, X. nematophilus exploited host carbohydrates for growth, indicating direct competition for host resources (Chapters 3 & 5). In vitro assays also revealed direct inhibition of bacterial growth by host haemolymph protein (Chapter 5). Altogether these results allude to a greater significance for the self-medication behaviour displayed by S. littoralis during infection.