A testable eco-evolutionary framework for zoonotic emergence.

Zoonotic spillover, the transmission of pathogens between animals and humans, is increasingly recognized as a major driver of emerging infectious diseases. However, most spillover research continues to approach emergence as a pathogen-specific or event-based phenomenon, limiting the ability to generalize findings across biological systems or to anticipate emergence before outbreaks occur. In this review, we build upon the existing eco-evolutionary framework to develop a pathogen-agnostic conceptual and operational framework that positions spillover as an emergent outcome of interacting ecological and evolutionary processes operating across hosts, environments, and time. The framework is organized around four interacting system-level pillars: pathogen evolvability, host network architecture, environmental mediation of persistence and exposure, and selective filtering and amplification under ecological and anthropogenic change. We propose that spillover risk emerges nonlinearly from the convergence of these interacting processes rather than from any single factor acting independently. Importantly, we move beyond conceptual synthesis by outlining measurable indicators and empirically testable approaches derived from genomics, ecology, environmental surveillance, and network analysis to operationalize the framework across diverse pathogen systems. Additionally, we discuss comparative and longitudinal study designs to evaluate spillover dynamics among viruses, bacteria, parasites, and other microbial threats. By reframing spillover as a dynamic eco-evolutionary system rather than an isolated transmission event, this review provides a comparative and operational foundation for studying zoonotic emergence and for informing surveillance, early warning, and intervention strategies amid accelerating environmental changes.