Emerging infectious diseases (EIDs) pose significant threats to wildlife and public health globally. This work explores the dynamics of EIDs through the interactions between the house finch (Haemorhous mexicanus) and the bacterial pathogen Mycoplasma gallisepticum (MG). The research focuses on understanding factors behind disease emergence and spread, the complex relationships between wildlife pathogens, and the implications for conservation biology and public health. By integrating molecular biology, ecology, and evolutionary biology, and combining field data with captive experiments, the study provides a comprehensive view of the host-pathogen interactions.The research reveals significant findings on the population genetics of house finches and their response to MG infection. It identifies genetic differentiation among populations, particularly within the native range in Mexico, suggesting varied responses to the pathogen and influencing disease dynamics. The study confirms the circulation of MG among Mexican wild birds, identifying multiple spillover events and variations in strains, marking the first report of MG in Mexican wildlife. Additionally, experimental and field results demonstrate that MG and Plasmodium coinfection in house finches shows a positive feedback, increasing disease severity for both pathogens. This dissertation underscores the importance of multidisciplinary approaches to study EIDs and the challenges they pose, emphasizing continuous monitoring and improved diagnostic tools for an effective disease control measures. By enhancing our understanding of host-pathogen interactions, this work aims to protect wildlife, conserve biodiversity, and safeguard public health.