Spillover

Spillover refers to the transfer of a phenomenon from one domain to another, producing effects beyond the original source. In epidemiology, spillover (often called zoonotic spillover or cross-species transmission) describes the process by which a pathogen maintained in an animal reservoir infects humans or other species, potentially leading to limited or sustained human transmission.

Mechanisms include increased contact between humans and wildlife or livestock, ecological disruption, and pathogen adaptation that expands host range. Environmental changes, climate shifts, habitat fragmentation, wildlife trade, and intensive agriculture can raise the chances of spillover by creating new interfaces or selective pressures for pathogens. Spillover is typically followed by onward transmission, sometimes leading to outbreaks or pandemics if the pathogen becomes efficiently human-adapted.

Beyond biology, spillover also appears in economics and policy. Knowledge spillovers occur when information or ideas move from one firm, sector, or country to others, boosting productivity without direct exchange. Negative spillovers or externalities arise when a decision imposes costs on others, such as pollution or systemic risk in financial markets.

Notable biological spillovers include HIV from chimpanzees and SARS-CoV-1 and SARS-CoV-2 linked to animal reservoirs. In policy contexts, spillovers complicate governance, as actions in one jurisdiction influence outcomes elsewhere.

Mitigation focuses on surveillance and rapid response, reducing risky human-animal interfaces, and applying a One Health approach that integrates human, animal, and environmental health. Economic spillovers can be managed with coordination, knowledge diffusion, and policies that internalize externalities.