A Primer on the “Spillover” of Zoonotic Infections

Spillover: Animal Infections and the Next Human Pandemic, by David Quammen (Amazon)

The purpose of this book is not to make you more worried. The purpose of this book is to make you more smart.

I bought this book as soon as COVID-19 appeared in the US for the above reason. I completely understand why people would want to avoid immersing in information about scary topics like deadly viruses while living through a pandemic and the uncertainties and anxieties it creates, but I felt like being armed with information was the only thing I could do for myself, and others, in addition to following standard guidelines. (For the love of everything, just wear a gd mask. Why is this even a political issue in America?)

Richard Preston’s Crisis in the Red Zone last year warned that, harrowing as its most recent outbreak was, an ebolavirus wasn’t going to be the “Next Big One” — that is, the next animal infection that’s transmissible to humans, or a zoonosis — to spill over between species and cause death on a massive scale. His predictions were, as I mentioned in my review then, eerie and unsettling. And entirely accurate.

Preston joins many scientists and epidemiologists who have warned similarly, and particularly hauntingly that the economic disparity inherent in places including the United States would be especially magnified when — not if — the Next Big One hits. Again, he wasn’t wrong. Understanding something about these specific infections, how disaster had been averted in previous spillovers (scarily, sometimes the answer is just “shrug”), and what we can learn from hard-won developments can only be beneficial as we look to reshape societies in the wake of the COVID-19 pandemic.

Prolific science writer David Quammen’s 2012 book Spillover is even more prophetic than Preston’s book. There are so many instances that point, overtly and subtly, to the inevitability of a severe spillover and how biblically destructive it would be (“How many other Nipahs are slouching toward Bethlehem to be born?“). It’s hard to believe that we were so collectively unprepared when science and experts have been clearly warning about this for so long, although Quammen also notes that of course “the degree of uncertainties is also high.”

What have we learned from the SARS experience? One thought that turns up… is that “humankind has had a lucky escape.” The scenario could have been very much worse. SARS in 2003 was an outbreak, not a global pandemic.

Spillover traces multiple zoonotic infections from what’s known, including their genesis in animals and the eventual, often curious jump to humans, and what this indicated about specific circumstances. He begins with Hendra, a rare but severe virus that passed from bats to horses and had a small jump to humans, before covering ebola, Lyme disease, Nipah, HIV, SARS, and other zoonotic viruses that spilled over.

This is information-heavy but not at all dense. Quammen has written extensively for popular science publications like National Geographic and that kind of accessible tone is what the book employs. It makes for not only interesting and even atmospheric reading, but it’s easily educational. By that I mean that I learned what I needed to without feeling overwhelmed or having to constantly catch myself up with outside research considering that I don’t have a good foundation in this science. It’s really the ideal primer in this area.

It’s also thorough in tracing the murky origins of various zoonotic illnesses, and guess what: humans are frequently responsible for bringing about our own disasters here: “Ecological disturbance causes diseases to emerge. Shake a tree, and things fall out.” Sometimes it’s as simple as that, sadly.

He’s able to break complex biological, epidemiological, and medical concepts down into understandable and relatable analogies. The result was that it made much of it less scary, which is what I had hoped for as a best possible outcome of reading this. This, for example, made a lot of sense to me and helped explain something I hadn’t completely understood around viruses making an interspecies leap: “Every spillover is like a sweepstakes ticket, bought by the pathogen, for the prize of a new and more grandiose existence. It’s a long-shot chance to transcend the dead end. To go where it hasn’t gone and be what it hasn’t been. Sometimes the bettor wins big. Think of HIV.”

Quammen explains that in order for a zoonosis to successfully establish in humans, we have to “consider two aspects of a virus in action: transmissibility and virulence”. Returning to the example of ebolavirus, it’s unlikely to have pandemic potential because of its transmissibility factor: a person has to come in direct contact with infected fluids as opposed to the easily spreadable droplets carrying coronaviruses. Quammen clearly put his money on the Next Big One likely being a SARS-coronavirus, one of many prophetic ideas here.

He traveled extensively in researching these cases, and the global aspect of the infections covered underscores how interconnected we all are, if current events haven’t driven that home sufficiently already: “The subject of animal disease and the subject of human disease are, as we’ll see, strands of one braided cord.” It was fascinating to watch mysteries get unraveled or puzzles solved, like in the concept of determining in past spillovers which animals were reservoir hosts and which were vectors in transmissions. Even just the concept of how that works was illuminating. For a popular science book, it’s a page-turner that also reads variously like a thriller, a mystery, rich nature writing, and epic adventure.

And, on a realistic level, it emphasized that what’s happening now is nature behaving as it always does, terrifying though that thought may be: “Although infectious disease can seem grisly and dreadful, under ordinary conditions it’s every bit as natural as what lions do to wildebeests and zebras, or what owls do to mice.”

There is hope in knowledge and preparation, and that’s why I think reading about this and the historical precedence of previous outbreaks is so important. We have been disappointingly mired in misinformation since the emergence of COVID-19, and taking responsibility to be better informed however possible is crucial. This is a gripping, highly informative, and clear-eyed look at zoonotic infections and although it doesn’t provide a wealth of actionable suggestions for the moment, it does give food for thought about what needs to be done, on a global health scale and ecologically, to prevent similar spreads in the future, especially considering zoonoses have become more prevalent recently.

Must-read of the year? Undoubtedly.

Animal Infections and the Next Human Pandemic

by David Quammen
published October 1, 2012 by W.W. Norton



20 thoughts on “A Primer on the “Spillover” of Zoonotic Infections

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  1. I loved this book and I’m so happy you did too. Great review! In light of what’s going on the world this book should be required reading. Here’s a link to my post from a few years ago:


    Another good one is Nathan Wolfe’s The Viral Storm: The Dawn of a New Pandemic Age. Here’s a link:


    Liked by 1 person

    1. I’m glad you liked it too! This book had managed to completely escape me somehow then when the outbreak began I felt like it was suddenly referenced everywhere. I see why after reading it. So informative and well written. I hadn’t heard of The Viral Storm, thanks for the tip about that one!

      Liked by 1 person

  2. A sweepstake for pathogens? Goodness what a thought ! But it’s true COVID won because of mankind’s greed and stupidity in thinking itself master of the universe and able to behave any old how with impunity.

    Liked by 1 person

    1. Isn’t it though?! It did make a lot more sense to me though, because I wondered why the leap was worth it when the virus was so deadly to its new hosts. It made so much sense the way he described it. And ugh, yes, our greed and stupidity knows no bounds and lately has taken it to a new low. This whole debate of personal freedom vs. science, sense, and logic has me in a constant rage.

      Liked by 1 person

    1. Thank you!! I’m glad I could get you to consider it. It’s not necessarily something I’d have rushed to read without being in the state we’re in, but I promise it’s exceptional. I read it while super busy with work (and of course stressed with quarantine, being in the epicenter of this nightmare in NYC, etc.) and instead of wanting to avoid thinking about it I looked forward to reading this each night like nothing else. It’s just so compelling!

      Liked by 1 person

  3. i already had this on my TBR (i imagine because of you, haha) but i’m definitely going to move it up. i agree with you that it’s helpful to understand scary things and how they work. and as someone who just started an MPH in epidemiology/biostats, i’m sure it’ll be even more relevant to me.

    Liked by 1 person

    1. Oh that’s fantastic that you’re starting that program! Of all times that we need more intelligent, informed people working in those fields too. You’re amazing!! And I definitely think this would be a must-read for you, especially now!

      Liked by 1 person

    1. And bats. I used to love watching the bats fly around at night at my family’s house in the country and now I wouldn’t want to be anywhere near them. Even their poo can be virus-riddled and dangerous!

      Liked by 1 person

      1. Bats just makes me think of that silly film Anchorman where there was that dodgy chicken fast food restaurant selling suspicious looking meat and the owner explained that it was “chicken of the cave”

        Liked by 1 person

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