RACSMEI addresses the high burden of infectious diseases in low- and middle-income countries, including Cambodia, where limited surveillance and laboratory capacity often obscure etiologies and transmission dynamics. This knowledge gap hinders the design of effective prevention and control strategies. RACSMEI will improve understanding across multiple pathogens using a multidisciplinary One Health approach. We will answer key questions on burden, ecology, transmission and population immune status to inform targeted and culturally appropriate interventions. The project combines a nationally representative One Health survey, social-science methods, and multiplex, diverse diagnostics to efficiently test for 57 priority pathogens, including zoonotic and vector-borne agents, vaccine-preventable and elimination-targeted diseases, enteric, respiratory, and environmentally transmitted pathogens and selected neglected tropical diseases and parasites relevant to Cambodia. Mathematical modelling will reconstruct and forecast transmission dynamics and assess the potential impact of future public-health strategies. By integrating intersectoral data and innovative methods, RACSMEI will generate actionable evidence for public-health authorities, support precision One Health interventions, and help reduce disease burden in affected communities. The project also aims to ensure the transferability of methods and insights to other countries facing similar challenges.