Throughout their lives, animals are faced with numerous ecological challenges stemming from abiotic and biotic conditions of their environment. Phenotypic shifts in response to one challenge can have cascading effects on other organismal systems, with downstream implications for individual fitness. Infectious disease presents a significant ecological challenge for most organisms on earth. Additionally, how an animal responds to disease can be shifted by exposure to other ecological challenges. Thus, infectious disease can both present an ecological challenge itself or shift as a consequence of another challenge. In this work, I used experimental captive studies on wild-caught house finches (Haemorhous mexicanus) to elucidate how an animal might shift its phenotypes when presented with an ecological challenge. In the first experiment, I examined how nutritional stress during nestling development impacted the magnitude of house finch responses to the bacterial pathogen Mycoplasma gallisepticum (MG). Although nutritional stress limited mass gain in nestlings, individual responses to MG did not vary with nutritional stress, possibly indicating that the development of immune responses is resilient even in the face of suboptimal nutritional conditions. Next, I investigated infectious disease as a challenge in itself and asked how individual social preferences were shifted by MG infection. I demonstrated that MG-infected house finches showed augmented sociality relative to control birds, choosing to spend more time with a group of conspecifics than alone. Because this increased social preference was no longer present once birds recovered, this phenotypic change in sociality may have specific benefits for actively infected birds. Finally, my last experiment expands upon these results, exploring whether group-living particularly benefits infected birds by offsetting two common fitness costs of infection: reduced foraging abilities and decreased anti-predator responses. Here we found that group-living provides all individuals with improved foraging and anti-predator behaviors, with the strongest benefits of group-living apparent for infected finches. This suggests that augmented sociality in infected house finches has important implications for surviving infection, and potentially, for the spread of MG within populations. As animals continue to face increasing and novel ecological challenges, it is vitally important to understand individual responses to environmental challenges, which can have long-term effects for all levels of biological organization. In particular, my work highlights the role of social behavior as a potentially adaptive phenotypic response to infectious disease in wild animals. Taken together, my results demonstrate the importance of continuing to study infectious disease from multiple perspectives to better understand how animals will respond to a shifting world. / Doctor of Philosophy / All animals must respond to challenges in their environment, which can impact their lives in a variety of ways. Infectious disease is a significant challenge for most organisms on earth. Infection with a disease-causing pathogen must be met by the individual with behavioral, physiological, and immunological responses to increase the animal's likelihood of survival. Additionally, an animal's response to disease can be shifted by exposure to other adverse environmental conditions, such as reduced access to food. On the one hand, infectious disease can present a challenge in itself. Alternatively, how an animal responds to disease may shift as a consequence of another challenge. In this work, I brought wild-caught birds into a captive setting and performed three experiments to determine how an animal might respond to common ecological challenges. First, I studied how food shortages during early life impacted how strongly birds responded to infection with a disease-causing bacteria. In this study I found that host responses to disease did not shift, even when birds were given less food and experienced reduced mass growth during early life. Although young animals are developing rapidly and are particularly vulnerable to challenges in their environment, my results indicate that the development of responses to disease is resilient even in the face of suboptimal conditions. Next, I investigated how social behaviors were shifted due to disease. Here I demonstrated that diseased birds were more social than healthy birds, preferring to spend more time with a group of other birds than alone. In contrast, once these same birds had recovered from infection and were again healthy they became less social, which suggests that diseased birds in particular may benefit from being part of a group. My final experiment expanded upon these results, exploring whether group-living can help increase an individual's survival by compensating for two consequences of disease: reduced ability to acquire food and evade predators. Here I found that group-living provides individual benefits in terms of both acquiring food and evading predators, both of which have important implications for an individual's survival, especially while experiencing disease. As animals continue to face increasing and new challenges due to global change, it becomes vitally important to understand individual responses to environmental changes. While the work highlighted here presents an important step in understanding individual responses, future work should use observational studies in the wild to determine how the social preferences and behaviors I demonstrated here are actually occurring in a natural habitat. Taken together, my results highlight the importance of continuing to study infectious disease from multiple perspectives to better understand how animals will respond to a shifting world.