Infection by multiple parasites is a part of everyday life for many organisms. The host immune system may be a central mediator of the many ways parasites might influence one another (and their hosts). Immunity provides a means for the colonized to reduce the success of current and future colonizers and has evolved across the tree of life several times independently. Along the way, the immune systems of plants as well as many groups of animals has evolved perhaps an accidental vulnerability wherein defense against one parasite can increase susceptibility to others. This so-called immune 'cross-talk' is a conundrum worth investigating not only to understand the impact of parasites on focal organisms, but also to better predict how immunity itself influences the evolution and epidemiology of parasites whose spread we might like to curtail. For plants, co-infection often comes from insect herbivores and various bacteria that colonize the leaf interior. Both colonizers can reduce plant fitness directly or indirectly by potentiating future enemies via cross-talk in plant immunity. This phenomenon has largely been studied in laboratory model plants, leaving a substantial gap in our knowledge from native species that interact in the wild. This dissertation helps close this gap by investigating the ecology of co-infection of a native plant by its major insect herbivore and diverse leaf-colonizing bacteria. I revealed that leaf co-infection in the field by leaf-mining herbivores and leaf-colonizing ("phyllosphere") bacteria is substantially more common than single infection by either group and that bacterial infection can cause increased feeding by herbivores in the laboratory. Immune cross-talk can also shape the field-scale patterns of herbivory across a native plant population. Studying the main herbivore of this native plant in detail revealed that, in contrast to many specialist herbivores, our focal species avoids plant defenses likely because it does not possess a specialized means of avoiding their toxicity. Nonetheless, this species may depend on the very same defenses it avoids by being initially attracted to plants that produce them. This foraging strategy is unique among known specialists. Lastly, I moved beyond immune cross-talk to explore how co-occurring phyllosphere bacteria might directly impact one another through competition. In the lab, I found that different growth strategies underlie competitive ability for two major clades of bacteria within the genus Pseudomonas, and that toxin production and resistance may be important mediators of competition within the phyllosphere. However, competitively superior bacteria that produce toxins may indirectly facilitate the survival of inferior competitors through their being toxin resistant, which likely enhances co-existence of diverse bacteria in the phyllosphere. Together, this dissertation has revealed a variety of means by which co-infecting bacteria and insects might influence one another through plant defense cross-talk, as well as how the complex interplay of colonization and competition might affect the structure of leaf microbial communities in nature.