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.

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.”

Take Advantage of Amazon's Early Black Friday Deals on Tech, Kitchen Appliances, and More


This article contains affiliate links to products selected by our editors. Mental Floss may receive a commission for purchases made through these links.

Even though Black Friday is still a few days away, Amazon is offering early deals on kitchen appliances, tech, video games, and plenty more. We will keep updating this page as sales come in, but for now, here are the best Amazon Black Friday sales to check out.


Instant Pot/Amazon

- Instant Pot Duo Plus 9-in-115 Quart Electric Pressure Cooker; $90 (save $40) 

- Le Creuset Enameled Cast Iron Signature Sauteuse 3.5 Quarts; $180 (save $120)

- KitchenAid KSMSFTA Sifter with Scale Attachment; $95 (save $75) 

- Keurig K-Mini Coffee Maker; $60 (save $20)

- Cuisinart Bread Maker; $88 (save $97)

- Anova Culinary Sous Vide Precision Cooker; $139 (save $60)

- Aicook Juicer Machine; $35 (save $15)

- JoyJolt Double Wall Insulated Espresso Mugs - Set of Two; $14 (save $10) 

- Longzon Silicone Stretch Lids - Set of 14; $13 (save $14)

HadinEEon Milk Frother; $37 (save $33)

Home Appliances


- iRobot Roomba 675 Robot Vacuum with Wi-Fi Connectivity; $179 (save $101)

- Fairywill Electric Toothbrush with Four Brush Heads; $19 (save $9)

- ASAKUKI 500ml Premium Essential Oil Diffuser; $22 (save $4)

- Facebook Portal Smart Video Calling 10 inch Touch Screen Display with Alexa; $129 (save $50)

- Bissell air320 Smart Air Purifier with HEPA and Carbon Filters; $280 (save $50)

Oscillating Quiet Cooling Fan Tower; $59 (save $31) 

TaoTronics PTC 1500W Fast Quiet Heating Ceramic Tower; $55 (save $10)

Vitamix 068051 FoodCycler 2 Liter Capacity; $300 (save $100)

AmazonBasics 8-Sheet Home Office Shredder; $33 (save $7)

Ring Video Doorbell; $70 (save $30) 

Video games


- Marvel's Spider-Man: Game of The Year Edition for PlayStation 4; $20 (save $20)

- Marvel's Avengers; $27 (save $33)

- Minecraft Dungeons Hero Edition for Nintendo Switch; $20 (save $10)

- The Last of Us Part II for PlayStation 4; $30 (save $30)

- LEGO Harry Potter: Collection; $15 (save $15)

- Ghost of Tsushima; $40 (save $20)

BioShock: The Collection; $20 (save $30)

The Sims 4; $20 (save $20)

God of War for PlayStation 4; $10 (save $10)

Days Gone for PlayStation 4; $20 (save $6)

Luigi's Mansion 3 for Nintendo Switch; $40 (save $20)

Computers and tablets


- Apple MacBook Air 13 inches with 256 GB; $899 (save $100)

- New Apple MacBook Pro 16 inches with 512 GB; $2149 (save $250) 

- Samsung Chromebook 4 Chrome OS 11.6 inches with 32 GB; $210 (save $20) 

- Microsoft Surface Laptop 3 with 13.5 inch Touch-Screen; $1200 (save $400)

- Lenovo ThinkPad T490 Laptop; $889 (save $111)

- Amazon Fire HD 10 Tablet (64GB); $120 (save $70)

- Amazon Fire HD 10 Kids Edition Tablet (32 GB); $130 (save $70)

- Samsung Galaxy Tab A 8 inches with 32 GB; $100 (save $50)

Apple iPad Mini (64 GB); $379 (save $20)

- Apple iMac 27 inches with 256 GB; $1649 (save $150)

- Vankyo MatrixPad S2 Tablet; $120 (save $10)

Tech, gadgets, and TVs


- Apple Watch Series 3 with GPS; $179 (save $20) 

- SAMSUNG 75-inch Class Crystal 4K Smart TV; $998 (save $200)

- Apple AirPods Pro; $169 (save $50)

- Nixplay 2K Smart Digital Picture Frame 9.7 Inch Silver; $238 (save $92)

- All-New Amazon Echo Dot with Clock and Alexa (4th Gen); $39 (save $21)

- MACTREM LED Ring Light 6" with Tripod Stand; $16 (save $3)

- Anker Soundcore Upgraded Bluetooth Speaker; $22 (save $8)

- Amazon Fire TV Stick with Alexa Voice Remote; $28 (save $12)

Canon EOS M50 Mirrorless Camera with EF-M 15-45mm Lens; $549 (save $100)

DR. J Professional HI-04 Mini Projector; $93 (save $37)

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How Much Is the Earth Worth?

The New York Public Library, Unsplash
The New York Public Library, Unsplash

Our home planet may be the most precious place we know, but it isn't priceless. The Earth's resources and the value it offers to humans add up to some unknown, tangible cost. The species may never have to worry about buying or selling the world, but thinking of it in terms of concrete numbers can help us better understand its value. Now, as Treehugger reports, one scientist has developed a special formula that allows us to do just that.

According to the calculations of Greg Laughlin, an assistant astronomy and astrophysics professor from the University of California, Santa Cruz, the Earth is worth roughly $5 quadrillion (or $5,000,000,000,000,000). He came up with that price after gauging the planet's mass, temperature, age, and other factors that directly correlate to its ability to sustain life.

To emphasize just how valuable the Earth is, Laughlin also estimated the worth of other planets in our solar system. Our nearest neighbor Mars costs about the same as a used car at $16,000. That's a fortune compared to Venus, which he appraised at the meager value of one cent.

Laughlin doesn't expect these numbers to have applications in the real world. Rather, he hopes they will inspire people to better appreciate the only home they know. He's not the first person to put a massive, hypothetical price tag on something just for fun. The cost of the Death Star from Star Wars has been calculated at $852 quadrillion—many times Laughlin's estimate for Earth.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.