Zoonotic Diseases 101: How Viruses Jump From Animals to Humans

A juvenile saddleback tamarin is measured as part of an annual health check of a population of three primate species in southeastern Peru.
A juvenile saddleback tamarin is measured as part of an annual health check of a population of three primate species in southeastern Peru.
ISHAAN RAGHUNANDAN

Though we don’t know exactly where the novel coronavirus originated, many scientists agree that it probably came from an animal. If that’s true, it means that COVID-19 is a zoonotic disease, or zoonosis—an illness caused by a pathogen that jumped from animals to humans.

The term zoonotic disease might not come up in regular conversation very often, but you surely know quite a few of them by name. Rabies, Lyme disease, AIDS, and plague are all known zoonoses, and scientists believe that Ebola virus disease and severe acute respiratory syndrome (SARS) originated from animal viruses, too. But that list just scratches the surface.

A 2017 study by researchers at the Centers for Disease Control and Prevention (CDC) estimated that 60 percent of the world’s known infectious diseases and up to 75 percent of new or emerging ones are zoonotic, and about 2.5 billion people are affected by a zoonotic illness each year (though only 2.7 million of those cases result in death).

Species and Spillovers

But for every pathogen that manages to escape one type of host and infect another (which is called a spillover event), there are countless others that can’t make the jump. One reason is that different species don’t mingle as closely as you may think.

“Don't shoot the messenger” definitely doesn't apply to zoonotic disease-bearing ticks.H_Barth/iStock via Getty Images

“To the casual observer, it might look like wildlife in native habitats are all mixed together and coming into close contact with one another, but in fact each species is compartmentalized into a particular ecological niche based on their feeding strategies and environmental requirements,” Bruce Rideout, director of disease investigations at San Diego Zoo Global, tells Mental Floss. “Each of these wildlife species will have an array of parasites or pathogens that have adapted to them, so these pathogens will also tend to be restricted to the ecological niche of the host. As long as ecosystems are intact, those pathogens will tend to stay in their native hosts and not spill over into others.”

According to Rideout, the rise in spillover events in recent decades is partially because humans are disrupting wildlife ecosystems more often. But even if you were to traipse through an undisturbed patch of forest and pet all the animals you see, it’s not a foregone conclusion that you’d fall ill.

For one thing, there’s a chance the pathogens wouldn’t make it into your body in the first place. Epidemiologist and veterinarian Julianne Meisner tells Mental Floss that sometimes “the type of contact needed for transmission isn’t something that would typically happen between an animal and a person.” It’s possible that the animal only transmits a certain virus from mother to offspring, through sexual intercourse, or via an insect that doesn’t bite humans.

The Perfect Storm

But even if one of the animals did have a virus that entered your body, it would still need to infiltrate your cells. To do this, it binds to the receptors on the surface of the cell, which then envelops all or part of the virus. Once inside, the virus hijacks the cell’s systems and uses them to manufacture more virus particles. However, if the virus can’t breach the cell walls in the first place, it can’t survive—and fortunately, many animal pathogens are specialist pathogens, which are only compatible with that species’ cell receptors. Generalist pathogens, on the other hand, are much more versatile.

“The greatest threat to humans is from generalist pathogens that have the ability to infect a wide range of hosts, either because they use cell surface receptors that are conserved across a wide range of species, or because they evolve rapidly and can quickly adapt to a new host,” Rideout says. The avian influenza viruses, for example, can adapt to infecting humans after mutating just once.

A backyard chicken maintains composure while getting vaccinated during a 2007 avian flu outbreak in Indonesia.Dimas Ardian/Getty Images

As National Geographic reports, there are other factors that impact an animal virus’s ability to cause an outbreak among humans, including how long the virus can survive without a host, how well the virus can thwart a human immune system, and how often humans come into contact with the species that carries the virus. In many cases, that perfect storm never happens, and a virus doesn’t progress beyond its first human host.

But increased human interference in wildlife ecosystems means more opportunities for generalist pathogens to jump to human hosts—and in order to predict which ones could cause the next outbreak or even pandemic, scientists have to first locate as-yet-undiscovered pathogens. Then, they study their behavior to identify those with the capacity to create that perfect storm. While there are various organizations that do this type of research at local, national, and international levels—the U.S. Agency for International Development’s PREDICT arm of the Emerging Pandemic Threats program, for example—the current pandemic has underscored the need for a greater global collaboration on this front.

It's a Small World

In a commentary published in the July 2020 issue of Science, the Wildlife Disease Surveillance Focus Group—a Washington University School of Medicine-affiliated coalition of infectious disease experts, ecologists, and other scientists—advocated for a decentralized, global database to store and share all research on animal pathogens.

“In the past, before modern transportation, spillover events would have been local and spread slowly, giving people elsewhere time to respond,” Jennifer A. Philips, co-director of Washington University’s division of infectious diseases and co-author of the article, said in a press release. “But now the world is so small that an event in one place puts the whole world at risk. This is not someone else's problem. It's everyone's problem.”

These proboscis monkeys are practically saying "You can't sit with us!" to other species.miskani/iStock via Getty Images

And preventing the next zoonotic disease pandemic isn’t only about surveillance and research—it’s also fundamentally linked to the preservation of the wildlife ecosystems themselves.

“The key thing for the general public to understand is that the best way to safeguard human health is to also safeguard wildlife and ecosystem health,” Rideout says. “The threat to us is not from wildlife; it is from the destruction of wildlife habitat and ecosystems … We need to shift our attention to long term sustainability.”

Mental Floss's Three-Day Sale Includes Deals on Apple AirPods, Sony Wireless Headphones, and More

Apple
Apple

During this weekend's three-day sale on the Mental Floss Shop, you'll find deep discounts on products like AirPods, Martha Stewart’s bestselling pressure cooker, and more. Check out the best deals below.

1. Apple AirPods Pro; $219

Apple

You may not know it by looking at them, but these tiny earbuds by Apple offer HDR sound, 30 hours of noise cancellation, and powerful bass, all through Bluetooth connectivity. These trendy, sleek AirPods will even read your messages and allow you to share your audio with another set of AirPods nearby.

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2. Sony Zx220bt Wireless On-Ear Bluetooth Headphones (Open Box - Like New); $35

Sony

For the listener who likes a traditional over-the-ear headphone, this set by Sony will give you all the same hands-free calling, extended battery power, and Bluetooth connectivity as their tiny earbud counterparts. They have a swivel folding design to make stashing them easy, a built-in microphone for voice commands and calls, and quality 1.18-inch dome drivers for dynamic sound quality.

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3. Sony Xb650bt Wireless On-Ear Bluetooth Headphones; $46

Sony

This Sony headphone model stands out for its extra bass and the 30 hours of battery life you get with each charge. And in between your favorite tracks, you can take hands-free calls and go seamlessly back into the music.

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4. Martha Stewart 8-quart Stainless-Steel Pressure Cooker; $65

Martha Stewart

If you’re thinking of taking the plunge and buying a new pressure cooker, this 8-quart model from Martha Stewart comes with 14 presets, a wire rack, a spoon, and a rice measuring cup to make delicious dinners using just one appliance.

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5. Jashen V18 350w Cordless Vacuum Cleaner; $180

Jashen

If you're obsessive about cleanliness, it's time to lose the vacuum cord and opt for this untethered model from JASHEN. Touting a 4.3-star rating from Amazon, the JASHEN cordless vacuum features a brushless motor with strong suction, noise optimization, and a convenient wall mount for charging and storage.

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6. Evachill Ev-500 Personal Air Conditioner; $65

Evachill

This EvaChill personal air conditioner is an eco-friendly way to cool yourself down in any room of the house. You can set it up at your work desk at home, and in just a few minutes, this portable cooling unit can drop the temperature by 59º. All you need to do is fill the water tank and plug in the USB cord.

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7. Gourmia Gcm7800 Brewdini 5-Cup Cold Brew Coffee Maker; $120

Gourmia

The perfect cup of cold brew can take up to 12 hours to prepare, but this Gourmia Cold Brew Coffee Maker can do the job in just a couple of minutes. It has a strong suction that speeds up brew time while preserving flavor in up to five cups of delicious cold brew at a time.

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8. Townew: The World's First Self-Sealing Trash Can; $90

Townew

Never deal with handling gross garbage again when you have this smart bin helping you in the kitchen. With one touch, the Townew will seal the full bag for easy removal. Once you grab the neatly sealed bag, the Townew will load in a new clean one on its own.

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FenSens

Parking sensors are amazing, but a lot of cars require a high trim to access them. You can easily upgrade your car—and parking skills—with this solar-powered parking sensor. It will give you audio and visual alerts through your phone for the perfect parking job every time.

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Noerden

Reusable water bottles are convenient and eco-friendly, but they’re super inconvenient to get inside to clean. This smart water bottle will clean itself with UV sterilization to eliminate 99.9 percent of viruses and bacteria. That’s what makes it clean, but the single-tap lid for temperature, hydration reminders, and an anti-leak functionality are what make it smart.

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Prices subject to change.

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100 Fascinating Facts About Earth

The best Spaceball.
The best Spaceball.
NASA

Did you know that there’s a place in the South Pacific Ocean called Point Nemo that’s farther from land than any other point on Earth? So far, in fact, that the closest humans are usually astronauts aboard the International Space Station. (And by the way: The map you’re about to look for Point Nemo on might not be entirely accurate; a certain amount of distortion occurs when trying to depict a 3D planet on a 2D surface.)

In this all-new episode of The List Show, Mental Floss editor-in-chief Erin McCarthy is journeying to the center of the Earth, and visiting its oceans, its atmosphere, and even space, in search of 100 facts about our endlessly fascinating planet.

The subjects that fall under the umbrella of “facts about Earth” are nearly as expansive as Earth itself. Geology, biology, astronomy, and cartography, are all fair game—and those are just a few of the many -ologies, -onomies, and -ographies you’ll learn about below. 

Press play to find out more Earth-shattering facts, and subscribe to the Mental Floss YouTube channel for more fact-filled videos here.