In wildlife markets across Laos, Finlayson’s squirrels are a common purchase, sold live for meat, stacked in cages at roadside stalls. Between 2014 and 2017, researchers sampled 359 animals across nine trade hotspots. Nearly half the Finlayson’s squirrels carried Leptospira , the bacterium that causes leptospirosis.

Buy three squirrels and the probability that at least one is infected is 83%. The hunters who trapped them, the vendors who handled them, the families who butchered them for dinner all touched the animals, the urine, the blood. None of them would have known.

Emerging infectious diseases, those that are new to human populations or rapidly expanding their range, are dominated by zoonoses . About 60% are transmitted from animals, and the majority of those come from wildlife. A quarter of all mammal species on Earth are now involved in legal and illegal trade. A new study in Science , led by Jérôme Gippet at the University of Lausanne, has quantified how that relates to new human diseases.

How an Ant Ecologist Found the Pattern

Gippet is an expert on invasive ants, not mammalian epidemiology. He wandered into the wildlife trade because a student wanted a project on mammal conservation and the topic sat at the intersection of their interests. Working with co-author Mattéo Schweizer, he combined 40 years of trade records (legal shipments logged by CITES, U.S. import data from Fish and Wildlife Service, and seizure records from illegal trafficking) with a global host-pathogen database covering more than 6,000 mammal species.

Early in the analysis, they hit a result that didn’t make sense. Species carrying zoonotic pathogens, meaning diseases that jump from animals to humans, were clearly overrepresented in trade. Yet their proportion was falling over time. Gippet spent a year on the puzzle. The answer, they believe, is that species new to trade haven’t been in contact with humans long enough for their pathogens to spill over and be detected. A pathogen only shows up in the data after it has successfully jumped to a human host and someone has documented it. As new species enter trade, they look “clean,” diluting the percentage of known zoonotic hosts even as the actual risk grows. Pathogen sharing isn’t a fixed biological trait. It’s acquired through sustained contact.

The headline numbers bear this out. Among traded mammals, the authors found, 41% share at least one pathogen with humans, compared with 6.4% of non-traded mammals. Much of that gap reflects the fact that traded species are also better-studied and more likely to live near people. But even after controlling for evolutionary relatedness, geography, research effort and diet, traded species are 1.5 times as likely to be zoonotic hosts. And there is a clear effect of time. For every decade a species has been present in the global wildlife trade, it shares one additional pathogen with humans. Species traded for 40 years carry four more shared pathogens than recent entrants. Live-animal markets amplify the effect. Species sold alive share 1.5 times more pathogens than those traded only as products.

Researchers in my own field have spent decades cataloguing the biological traits that make certain species dangerous reservoirs. Diet, immune architecture, behavioral characteristics such as colony size in bats . That work has produced real insights. It helps identify which animals carry pathogens most likely to be lethal in humans, it sharpens surveillance by narrowing the search to the highest-risk hosts, and it informs our understanding of how new diseases evolve. But Gippet’s data suggest that the dominant variable may be ecological rather than biological. Gippet sees the same pattern in his home discipline, the study of how species establish themselves in new environments. Researchers spent decades searching for traits that predict which organisms become successful invaders. The most powerful predictor turned out to be propagule pressure, the sheer number of introduction attempts. “I really try to think in terms of first dispersal,” Gippet told me. Once you’ve accounted for how many times a species gets introduced, he argues, you can then ask which local conditions allow it to persist. The same logic applies to pathogens. Contact comes first. Biology determines what happens next.

Matt Robinson, a molecular bacteriologist at the Lao-Oxford-Mahosot Hospital-Wellcome Trust Research Unit whose team generated those Laotian squirrel data, sees the same logic from the field. He calls it a numbers game. Volume matters alongside duration. Squirrels move through Laotian markets in large quantities, and a high-volume species traded for a few years may pose a greater near-term threat than a rare one traded for decades. Robinson also suspects that animal-to-human transmission happens more often than anyone measures. Most spillovers are dead ends. Symptoms resolve, or a patient gets treated for an undiagnosed fever and recovers. For every documented spillover, there may be hundreds that burned out in a single person and were never recorded.

The Costs Nobody Is Counting

Wildlife trade provides real value. Communities around the world depend on wild protein. Village-level hunting, household consumption and informal cross-border markets are all part of that picture, and Robinson, who works in Laos, emphasizes that blanket prohibitions ignore the food security they provide. At the other end of the spectrum is the pet trade, which carries no food security justification at all. In 2003, Gambian pouched rats shipped from Ghana to Texas were housed alongside prairie dogs at a pet distribution facility in Illinois. Mpox virus, a relative of smallpox that causes fever, body aches and a distinctive rash of painful fluid-filled lesions, jumped species. The prairie dogs were sold across the Midwest. Seventy-one people in six states developed symptoms. It was the first mpox outbreak ever recorded in the Western Hemisphere, produced entirely by a pet supply chain nobody was monitoring for disease.

Whether the trade feeds families or stocks pet shops, every transaction carries a disease externality that nobody is pricing. Local and regional trade, the kind Robinson samples in Laos, is invisible to the international datasets Gippet analyzed. Illegal trade amplifies risk further. “No trade is safe,” Gippet told me. “If we trade wild animals, we will create the opportunities for pathogen transmissions.”

CITES, the international framework that governs wildlife trade, exists to prevent extinction, not disease. The WHO Pandemic Agreement , adopted last year, begins to integrate health risks into trade governance, but its provisions remain untested. Gippet’s framing is economic rather than prohibitionist. Where trade supports livelihoods, he argues for investing in the monitoring, biosecurity and regulation we already apply to livestock. The cost-benefit analysis that would guide those decisions, weighing the economic and nutritional value of specific trades against their disease risk, has never been done.