** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** INFECTIOUS DISEASE EMERGENCE IS A CLEAR AND IMMINENT THREAT TO HUMANITY. BIODIVERSITY LOSS HAS BEEN IMPLICATED AS A CENTRAL DRIVER OF THE GLOBAL INCREASE IN DISEASE EMERGENCE, BUT THE DIRECTION AND MECHANISMS THAT CONNECT BIODIVERSITY WITH DISEASE EMERGENCE ARE CONTROVERSIAL.HERE WE OFFER A NOVEL PERSPECTIVE, HYPOTHESIZING THAT THE LOSS OF PATHOGEN BIODIVERSITY MAY CONTRIBUTE TO DISEASE EMERGENCE RISK. WE POSIT THAT EXPOSURE TO DIVERSE RELATED PATHOGENS PROMPTS HOSTS TO ESTABLISH MULTIVALENT CROSS-REACTIVE ANTIBODY PORTFOLIOS, WHICH CAN (I) PREVENT SEVERE DISEASE, (II) REDUCE TRANSMISSION, AND ULTIMATELY (III) LIMIT THE RISK OF PATHOGEN EMERGENCE. IF TRUE, THESE INSIGHTS COULD BE APPLIED TODESIGN MULTIVALENT PORTFOLIO VACCINES, WHICH CAN PROVIDE RELATIVELY STABLE PROTECTION AGAINST BROAD VIRAL LINEAGES. WE WILLCOMBINE MECHANISTIC MATHEMATICAL MODELS WITH EXPERIMENTAL AND OBSERVATIONAL FIELD STUDIES TO TEST THESE IDEAS, USING EAST AFRICAN NAIROVIRUSES AS A MODEL SYSTEM, WHICH INCLUDE IMPORTANT ANIMAL (E.G. NAIROBI SHEEP DISEASE VIRUS, NSDV) AND ZOONOTIC (E.G. CRIMEAN-CONGO HAEMORRHAGIC FEVER VIRUS, CCHFV) PATHOGENS.