Pandemic Viruses Not Specially Adapted Before Infecting Humans, Study Finds

UC San Diego researchers challenge long-held assumption about how animal viruses spark human outbreaks.

Published on Mar. 10, 2026

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A new study published in Cell challenges the prevailing assumption that animal viruses must first acquire special evolutionary adaptations before they can spark human epidemics and pandemics. Analyzing viral genomes from outbreaks caused by influenza A, Ebola, Marburg, mpox, SARS-CoV, and SARS-CoV-2, researchers found no evidence of pre-spillover adaptation in these viruses. Instead, measurable evolutionary changes typically appeared only after sustained transmission began in people.

Why it matters

The findings have direct relevance to the ongoing debate around the origins of COVID-19, as they suggest SARS-CoV-2 was not shaped by selection in a laboratory or prolonged evolution in an intermediate host prior to its emergence. More broadly, the results challenge the idea that pandemic viruses are evolutionarily special before reaching humans, indicating many may already possess the basic capacity to infect and spread among people.

The details

The research team used a sophisticated phylogenetic framework to analyze changes in natural selection across entire viral genomes, comparing rates of different mutations. This allowed them to detect whether selection was intensified, relaxed, or unchanged as viruses transitioned from animal reservoirs to human outbreaks. Importantly, the approach was validated using known examples of laboratory-adapted or artificially selected viruses, which produced distinct evolutionary signatures.

  • The study was published on March 10, 2026 in the journal Cell.

The players

Joel Wertheim

Senior author of the study and professor of medicine in the Division of Infectious Diseases and Global Public Health at UC San Diego School of Medicine.

University of California San Diego

The institution where the study was conducted.

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What they’re saying

“This work has direct relevance to the ongoing controversy around COVID-19 origins. From an evolutionary perspective, we find no evidence that SARS-CoV-2 was shaped by selection in a laboratory or prolonged evolution in an intermediate host prior to its emergence. That absence of evidence is exactly what we would expect from a natural zoonotic event — and it represents another nail in the coffin for theories invoking laboratory manipulation.”

— Joel Wertheim, Professor of medicine (Mirage News)

“From a broad epidemiological standpoint, our findings challenge the idea that pandemic viruses are evolutionarily special before they reach humans. Rather than requiring rare, finely tuned adaptations in animals, many viruses may already possess the basic capacity to infect and transmit between humans. What matters most is human exposure to a diverse array of animal viruses.”

— Joel Wertheim, Professor of medicine (Mirage News)

What’s next

The researchers see potential applications of their framework in outbreak forensics, viral surveillance, and pandemic preparedness, to better understand how pandemics begin and focus efforts on prevention.

The takeaway

This study challenges the long-held assumption that animal viruses must first undergo special evolutionary adaptations before they can spark human epidemics and pandemics. The findings suggest many viruses may already possess the basic capacity to infect and spread among people, underscoring the importance of reducing human exposure to diverse animal viruses.