Recognizing the importance of emerging infectious diseases (EIDs), the World Health Organization (WHO) highlighted Disease X, a currently unknown pathogen capable of causing a serious human epidemic, as a target for research and development in their 2018 Blueprint of Priority Diseases ( 1). A single lethal virus can emerge suddenly and spread rapidly to every household and every community without regard to national borders or to social and economic standing.

We now live in an era in which threats posed by viral pandemics are a daily reality. SpillOver is a living, interactive database that can be refined over time to continue to improve the quality and public availability of information on viral threats to human health. The publicly accessible SpillOver platform can be used by policy makers and health scientists to inform research and public health interventions for prevention and rapid control of disease outbreaks.

Using a scientifically informed process, we capitalized on the recent wealth of virus discovery data to systematically identify and prioritize targets for investigation. Several newly detected wildlife viruses ranked higher than known zoonotic viruses. Validating the risk assessment, the top 12 were known zoonotic viruses, including SARS-CoV-2. Using data from testing 509,721 samples from 74,635 animals as part of a virus discovery project and public records of virus detections around the world, we ranked the spillover potential of 887 wildlife viruses.

We then developed a risk ranking framework and interactive web tool, SpillOver, that estimates a risk score for wildlife-origin viruses, creating a comparative risk assessment of viruses with uncharacterized zoonotic spillover potential alongside those already known to be zoonotic. Using expert opinion and scientific evidence, we identified host, viral, and environmental risk factors contributing to zoonotic virus spillover and spread in humans. Strategies are needed to identify and characterize animal viruses that pose the greatest risk of spillover and spread in humans and inform public health interventions. The death toll and economic loss resulting from the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic are stark reminders that we are vulnerable to zoonotic viral threats.

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