Roughly 75% of emerging infectious diseases in humans originate in animals. That statistic, drawn from a landmark 2008 analysis in Nature that cataloged over 300 emerging disease events between 1940 and 2004, has held up through repeated study. HIV came from chimpanzees. SARS-CoV-2 almost certainly came from bats. The next pandemic will very likely come from an animal too.
A zoonotic disease is any infection that jumps from a non-human animal to a human host. The jump itself is called spillover. Most spillover events go nowhere. A single person gets sick, doesn't transmit onward, and the chain dies. But occasionally a pathogen lands in the right host, adapts quickly enough to spread between people, and triggers an outbreak that can scale to a pandemic.
Which animals pose the greatest risk?
Bats, rodents, primates, and livestock account for the vast majority of zoonotic spillover events. Bats carry an extraordinary viral diversity (over 200 known coronaviruses alone) and their unique immune systems allow them to harbor viruses without becoming ill, creating enormous reservoir populations.
Rodents are everywhere humans are. They spread hantaviruses, Lassa fever, and plague, often through contaminated droppings in and around human dwellings. Primates share so much genetic similarity with us that their pathogens need minimal adaptation to infect human cells. HIV crossed from chimpanzees to humans through bushmeat hunting in central Africa, probably in the early 20th century.
Livestock are a different kind of risk. Factory farms concentrate thousands of genetically similar animals in tight quarters, creating ideal conditions for a virus to amplify and mutate. The ongoing H5N1 avian influenza outbreak jumped from wild birds to commercial poultry, then into US dairy cattle herds in 2024, bringing it into regular contact with farmworkers. Pigs are particularly dangerous as "mixing vessels" because they have receptors for both avian and human influenza viruses, allowing genetic reassortment that produces entirely new strains.
How does spillover actually happen?
A virus can't just decide to infect a new species. It needs physical access to a human host, the molecular machinery to enter human cells, and the ability to replicate once inside. Spillover is a numbers game: the more contact between humans and animal reservoirs, the more chances a pathogen gets to clear those hurdles.
Five main pathways create that contact.
Bushmeat hunting and butchering. Handling raw wild animal carcasses exposes hunters to blood and bodily fluids. Ebola outbreaks in central Africa have been repeatedly traced to contact with infected bat or primate carcasses. The 2014 West Africa outbreak that killed over 11,000 people likely started with a 2-year-old child exposed to bats in Guinea.
Live animal markets. Markets where multiple wild and domestic species are caged in close proximity, often stacked on top of each other, create a mixing environment where viruses can jump between species. The original SARS-CoV emerged from a live animal market in Guangdong, China in 2002, with palm civets as the likely intermediate host between bats and humans.
Agricultural intensification. Nipah virus first emerged in Malaysia in 1998 when fruit bats, displaced by deforestation, began feeding in mango trees planted alongside pig farms. Bats dropped partially eaten fruit contaminated with their saliva into pig enclosures. Pigs became infected, amplified the virus, and transmitted it to 265 farmworkers. 105 of them died. That's a 40% case fatality rate.
Deforestation and land-use change. Clearing tropical forest pushes humans directly into habitats teeming with bat colonies, rodent populations, and primate groups. A 2015 EcoHealth Alliance study found that deforestation hotspots in Southeast Asia, West Africa, and the Amazon Basin correlated strongly with emerging disease events.
Pet and wildlife trade. The 2003 monkeypox outbreak in the US was traced to Gambian pouched rats imported from Ghana, which infected co-housed prairie dogs at a pet distributor in Illinois. 47 people across 6 states were infected.
Why is spillover accelerating?
Between 1940 and 2004, emerging infectious disease events increased roughly fourfold per decade. That trend hasn't reversed. Several forces are pushing the rate higher.
Deforestation claims approximately 10 million hectares of tropical forest annually, according to FAO data. Each hectare cleared brings humans closer to animal reservoirs that previously had minimal contact with people. Climate change is shifting the geographic ranges of disease-carrying species (bats, mosquitoes, ticks) into new territories, bringing their pathogens with them.
Global livestock production has tripled since 1970. Larger herds in more concentrated facilities mean more opportunities for pathogens to amplify and adapt to mammalian hosts before encountering humans. When H5N1 spread through US dairy cattle in 2024, it demonstrated exactly this risk: a bird virus gaining new mammalian adaptations in a livestock population that interacts daily with farmworkers.
International wildlife trade moves millions of animals across borders every year, transporting their microbial passengers along with them. Legal trade is tracked imperfectly. Illegal trade isn't tracked at all.
What are the most dangerous zoonotic pathogens right now?
Not all zoonotic diseases carry pandemic potential. Rabies kills roughly 59,000 people annually but doesn't transmit efficiently from human to human. The ones that matter most for pandemic risk combine animal origin with proven or plausible human-to-human transmission.
H5N1 avian influenza tops most risk lists. Its reported CFR among confirmed human cases exceeds 50%, though that figure is almost certainly inflated by testing bias (see CFR vs IFR). What makes H5N1 alarming is its steady acquisition of mammalian adaptations through circulation in dairy cattle and other mammals. If it gains efficient airborne transmission between humans, the consequences could be severe.
Nipah virus has a CFR between 40% and 75% depending on the outbreak, no approved vaccine, and no specific treatment. Bangladesh experiences nearly annual Nipah outbreaks, typically linked to date palm sap contaminated by bat droppings. Human-to-human transmission occurs but has so far remained limited to close contacts.
Novel coronaviruses remain a top concern because the coronavirus family has produced three major outbreaks since 2002 (SARS, MERS, COVID-19) and bats harbor hundreds of additional coronaviruses that haven't yet spilled over.
What is the One Health approach?
One Health recognizes that human health, animal health, and environmental health are interconnected. You can't prevent zoonotic pandemics by focusing on human medicine alone. You need veterinary surveillance to catch animal outbreaks before they reach people, environmental monitoring to identify high-risk interfaces where spillover is likely, and coordinated response systems that span all three domains.
In practice, One Health means testing bat populations near human settlements for novel viruses. It means monitoring poultry and livestock herds for emerging strains. It means preserving intact ecosystems that act as buffers between wildlife reservoirs and human communities. And it means sharing surveillance data across agencies that historically don't talk to each other.
WHO, FAO, and the World Organisation for Animal Health (WOAH) launched a joint One Health initiative in 2010. Progress has been slow. National veterinary surveillance systems remain underfunded in most countries, and wildlife monitoring is nearly nonexistent outside a handful of research programs.
What can you do?
You can't stop zoonotic spillover by yourself. But you can make better decisions with the right information.
When PandemicAlarm flags novel pathogens, check the origin. Animal-origin pathogens with evidence of human-to-human transmission deserve more attention than those without. If you're traveling to regions with active zoonotic outbreaks, consult our disease risks by region page for specific precautions.
Avoid direct contact with wild animals, especially bats, rodents, and primates. If you work with livestock, follow biosecurity protocols and report unusual animal illness to veterinary authorities. These aren't dramatic actions. They're the basic interface management that reduces your personal exposure to spillover risk.
Zoonotic spillover isn't going to stop. The conditions driving it are intensifying. What can change is how quickly we detect it when it happens, and how prepared you are when it does.