Is COVID-19 Considered a Zoonotic Disease?

COVID-19 is broadly classified as a disease of zoonotic origin, meaning the virus that causes it, SARS-CoV-2, almost certainly jumped from animals to humans before becoming a pandemic. That said, the label “zoonotic disease” is more contested than you might expect. Once the virus began spreading explosively between people, the ongoing pandemic was driven entirely by human-to-human transmission, which has led some researchers to argue it should be reclassified as an emerging infectious disease of probable animal origin rather than a true zoonosis. The distinction matters more than it sounds, and the animal side of the story has only gotten more complicated since 2020.

What “Zoonotic” Actually Means and Why COVID-19 Sits on the Boundary

The World Health Organization defines zoonoses as diseases or infections naturally transmitted between vertebrate animals and humans. Under that definition, a zoonotic disease involves repeated or ongoing animal-to-human transmission as part of its normal ecology. Rabies is a classic example: every human case comes from an animal bite. Avian influenza strains that occasionally infect poultry workers fit the pattern too. The key feature is that animals remain a regular source of human infection.

COVID-19 doesn’t quite fit that mold. While the initial emergence of SARS-CoV-2 almost certainly involved an animal-to-human spillover event, the more than 40 million infections documented in the early months of the pandemic were spread exclusively person to person. A 2020 analysis in Frontiers in Public Health argued that SARS-CoV-2 and COVID-19 do not meet the WHO definition of zoonoses and suggested the virus be reclassified as an emerging infectious disease of probable animal origin.¹ In practice, most virologists and public health agencies still refer to COVID-19 as zoonotic in origin, even if sustained transmission is now entirely between humans. The distinction is real but largely academic: what everyone agrees on is that SARS-CoV-2 came from animals.

Tracing the Virus Back to Bats

The closest known relatives of SARS-CoV-2 are coronaviruses found in horseshoe bats. Two bat viruses stand out. One called RaTG13, found in a Rhinolophus bat in Yunnan province, China, shares about 96.1% of its genome with SARS-CoV-2. Another called BANAL-52, sampled in Laos, is slightly closer at roughly 96.8% nucleotide identity and is particularly similar in the part of the spike protein that attaches to human cells.² Additional SARS-CoV-2-related coronaviruses have turned up in Rhinolophus shameli bats sampled in Cambodia as far back as 2010, sharing about 92.6% nucleotide identity with the pandemic virus.³

These numbers may sound impressively high, but a few percentage points of genetic difference across a full coronavirus genome represents decades of evolutionary divergence. A 2025 study in Cell used phylogenetic analysis while accounting for recombination among bat sarbecoviruses and estimated that the closest-inferred bat virus ancestor of SARS-CoV-2 existed less than a decade before the virus emerged in humans.⁴ That timeline is significant: it suggests the virus was circulating and evolving in animals for years before 2019, probably passing through one or more species along the way.

The Search for an Intermediate Host

If bats are the ancestral reservoir, something likely carried the virus closer to humans before the first spillover. For SARS (the 2003 outbreak), that intermediate host turned out to be palm civets sold in live-animal markets. For MERS, it was dromedary camels. For SARS-CoV-2, the question remains open.

Pangolins were early candidates. Coronaviruses found in Malayan pangolins share about 85 to 92% of their genome with SARS-CoV-2, making them the second-closest known relatives after bat coronaviruses. More striking, the receptor-binding domain of one pangolin coronavirus sub-lineage shares about 97.4% amino acid similarity with that of SARS-CoV-2, and pangolin coronaviruses contain all six key mutations thought to shape binding to human cells.⁵ Laboratory work confirmed that a pangolin coronavirus can infect hamsters through direct contact and that infected hamsters exhale infectious viral aerosols, highlighting the public health risk of these viruses circulating in trafficked wildlife.⁶

Still, the overall genome similarity between pangolin coronaviruses and SARS-CoV-2 is too low for pangolins to be the direct progenitor. The high similarity in the receptor-binding domain could be the result of recombination, where chunks of genetic material swap between coronaviruses when two strains infect the same animal. Pangolins may have played a role in the evolutionary history of SARS-CoV-2’s spike protein without being the animal that handed the fully formed virus to humans.

What the Huanan Seafood Market Tells Us

The earliest known cluster of COVID-19 cases was linked to the Huanan Seafood Market in Wuhan, which sold live wild animals alongside seafood and other goods. Environmental samples collected from the market after it was shut down in January 2020 tested positive for SARS-CoV-2, and detailed genetic analysis of those samples has provided some of the strongest circumstantial evidence for a zoonotic spillover.

A 2024 study in Cell examined SARS-CoV-2-positive samples from a specific wildlife stall at the market and found DNA from raccoon dogs, hoary bamboo rats, civets, hedgehogs, marmots, and other species in the same samples.⁷ Raccoon dogs are documented as susceptible to SARS-CoV-2 and have been experimentally confirmed as capable of transmitting it. The researchers also recovered animal viruses that infect raccoon dogs, civets, and bamboo rats from the same stall, confirming these species were present and alive at the market in late 2019.

A separate surveillance effort published in Nature analyzed both positive and negative environmental samples from the market using RNA sequencing. Bacteria dominated most samples, but mammalian DNA was widespread, consistent with what you would expect from a wet market environment. Beyond domestic species like chickens, ducks, pigs, and cattle, the researchers identified dozens of mammal genera.⁸ None of this proves an animal at the market infected a human, but the co-location of susceptible wildlife species and early SARS-CoV-2 genetic material in the same physical stall is the kind of evidence epidemiologists look for when tracing a spillover event.

Why the Virus Binds So Well to Human Cells

One of the puzzling features of SARS-CoV-2 is how effectively it attaches to human ACE2, the cell-surface receptor the virus uses to enter cells. A computational study that compared binding affinities across species found that the SARS-CoV-2 spike protein bound more tightly to human ACE2 than to that of any other species tested, with the ranking running from human at the top down through pangolin, dog, monkey, hamster, ferret, cat, tiger, bat, civet, and others.⁹ This result initially fueled speculation about the virus being engineered for humans, though the finding is also consistent with a virus that adapted through natural selection in an intermediate host whose ACE2 receptor happens to resemble the human version.

A broader analysis of ACE2 sequences from 410 vertebrate species, including 252 mammals, found that only mammals scored in the medium to very high range for predicted susceptibility to SARS-CoV-2, and only catarrhine primates (the group that includes humans, apes, and Old World monkeys) scored in the very high category.¹⁰ This doesn’t mean the virus was designed for primates. It means the molecular lock-and-key fit between SARS-CoV-2 and human ACE2 is unusually tight, which could have resulted from gradual adaptation in an animal whose receptor was close enough to ours to drive selection in that direction.

Adaptation to a new host also involves more than just the spike protein. Research on mouse-adapted SARS-CoV-2 has shown that mutations in the spike protein alone cause only mild infection. Full virulence in mice required additional mutations in nonstructural proteins involved in viral replication, suggesting that cross-species adaptation is a multi-gene process that takes time.¹¹

Humans Gave It Back to Animals

The zoonotic story of COVID-19 runs in both directions. Once the virus was circulating widely in people, humans became the main source of infection for animals rather than the other way around. A community-based study of pets in households with confirmed COVID-19 found that about 40% of dogs and 43% of cats tested seropositive for SARS-CoV-2 antibodies, while roughly 5% of dogs and 8% of cats were actively PCR-positive at the time of sampling.¹² Most naturally occurring infections in animals were presumed to have resulted from close contact with COVID-19 patients.¹³

This reverse flow of the virus, sometimes called anthroponosis or reverse zoonosis, raised concerns early in the pandemic. Researchers warned that if SARS-CoV-2 established itself in domestic or wild animal populations, those populations could serve as ongoing reservoirs, potentially generating new variants and spilling the virus back to humans in the future.¹⁴ That concern turned out to be well-founded.

Mink Farms as a Two-Way Street

The clearest documented case of SARS-CoV-2 bouncing from humans to animals and back again involved mink farms in the Netherlands. Outbreaks swept through densely housed mink populations after infected farmworkers introduced the virus. Whole-genome sequencing of virus samples from humans on and around those farms revealed strains with animal-specific sequence signatures, providing direct evidence that the virus had jumped from mink back into people.¹⁵

The mink situation was alarming enough that Denmark culled its entire farmed mink population, roughly 17 million animals, in late 2020. The concern was not just that mink could spread the virus to farmworkers; it was that rapid transmission through densely packed animal populations could accelerate viral evolution, producing variants with mutations that might help the virus evade human immunity. The mink episode became a textbook example of why the human-animal interface matters for pandemic management.

White-Tailed Deer and the Problem of Wildlife Reservoirs

Perhaps the most unexpected chapter in the animal side of COVID-19 involves white-tailed deer in North America. These deer are abundant, live in close proximity to human communities, and turned out to be highly susceptible to SARS-CoV-2. Surveillance studies found that the virus wasn’t just infecting individual deer; it was spreading among them and persisting long after specific variants had stopped circulating in people.

A study tracking SARS-CoV-2 in white-tailed deer in Pennsylvania from 2021 to 2024 found Alpha and Delta variants persisting in deer populations for months to years after those lineages had largely disappeared from human circulation, establishing deer as a potential reservoir for ancestral virus lineages.¹⁶ Separate work in New York state confirmed the co-circulation of Alpha, Gamma, and Delta variants in deer, with evidence of multiple spillover events from humans to deer followed by sustained deer-to-deer transmission and adaptation.¹⁷

This matters because a large wildlife reservoir gives the virus opportunities to evolve independently of human immune pressure. A variant that has been extinct in humans for years could theoretically re-emerge from a deer population, carrying mutations accumulated during its time in a non-human host. Whether this has actually happened is not yet clear, but the possibility keeps wildlife surveillance on the agenda.

COVID-19 Is Not Unique Among Coronaviruses

If the animal origin of SARS-CoV-2 sounds extraordinary, it is worth stepping back to see how ordinary it actually is among coronaviruses. Every human coronavirus for which we have good evolutionary data appears to have come from animals. MERS coronavirus, which still causes sporadic outbreaks in the Middle East, is considered a zoonosis with bats as the likely ancestral reservoir and dromedary camels as the intermediate host that regularly transmits it to people.¹⁸ The original SARS coronavirus jumped from bats through civets in 2002-2003.

Even the four “common cold” coronaviruses that circulate year-round have animal ancestors. Phylogenetic evidence indicates that two of them, HCoV-NL63 and HCoV-229E, originated from bat coronaviruses, while the other two, HCoV-OC43 and HCoV-HKU1, trace back to rodent coronaviruses.¹⁹ Camelids have also been suspected as an intermediate host for HCoV-229E. In other words, every coronavirus that infects humans appears to have started in an animal. SARS-CoV-2 follows the pattern perfectly, even if the specific details of its spillover are still being pieced together.

Why the One Health Framework Keeps Coming Up

The pandemic prompted a surge of interest in what public health professionals call the “One Health” approach, the idea that human health, animal health, and environmental conditions are fundamentally interconnected and need to be monitored together. The logic is straightforward: if the next pandemic virus is likely to come from an animal, you need veterinarians and wildlife biologists at the table alongside epidemiologists and physicians.

Researchers have proposed three primary targets for pandemic prevention grounded in this framework: surveillance at the interfaces where wildlife, livestock, and humans overlap; research to speed up vaccine and therapeutic development; and strategies to reduce the underlying drivers of spillover risk, including habitat destruction, the wildlife trade, and intensive animal farming.²⁰ A 2025 review in Nature Reviews Microbiology emphasized that risk-based One Health surveillance is critical for understanding spillover pathways and identifying where the risk is highest, integrating data across human, animal, and environmental sectors.²¹

The COVID-19 pandemic exposed how disconnected these sectors were in practice. Wildlife markets were poorly regulated, veterinary surveillance of coronaviruses in bat populations was underfunded, and when the pandemic hit, environmental sampling at the Huanan Market was conducted only after the market had been shut down and partially disinfected. The lesson, reinforced by researchers writing in the Journal of Preventive Medicine and Hygiene, is that an enhanced One Health approach with robust, interconnected global health strategies is urgently needed to prevent the next zoonotic crisis.²²

The Lab Leak Debate and What It Changes

No discussion of COVID-19’s origins is complete without acknowledging the competing hypothesis that SARS-CoV-2 may have accidentally leaked from a laboratory, specifically the Wuhan Institute of Virology, which conducted research on bat coronaviruses. This hypothesis has been taken seriously by intelligence agencies and some scientists, and as of 2025, investigations have not produced a definitive resolution either way.

It is worth being precise about what the lab leak hypothesis does and does not change about the zoonotic question. Even under the lab leak scenario, the virus itself still has animal origins; it would still be a bat-derived sarbecovirus. The disagreement is about the pathway: did the virus reach humans through natural contact with an infected animal (or a chain of infected animals), or did it reach humans through a laboratory incident involving a naturally collected or experimentally modified virus? Both pathways begin with animals. The question is whether the final step was a wildlife market or a biosafety failure.

For the public, the practical implications are subtly different. If the spillover happened naturally at a market, the policy response centers on wildlife trade regulation, market hygiene, and habitat management. If it happened through a lab incident, the response centers on biosafety protocols, gain-of-function research oversight, and laboratory transparency. Both responses are probably needed regardless of which origin story turns out to be correct, since natural spillovers and lab accidents are both real risks. The animal ancestry of the virus is not in serious dispute from either side of the debate.