What are the risks of a global pandemic?

quammen

David Quammen’s latest book Spillover: Animal Infections and the Next Human Pandemic is wonderfully written. As expected from someone the New York Times says “is not just among our best science writers but among our best writers, period.” And the Times is right, Quammen is awesome. I loved his widely praised The Song of the Dodo: Island Biogeography in an Age of Extinction. A mashup of nature travel, science and adventure that really works.The place to start if you haven’t read Quammen. Spillover uses the same template, but is not nearly as good. It’s drags and could have been cut by a third. But worse is the alarmist take on the risks of a pandemic. Some historical and evolutionary context will explain my reaction.

As expected, Quammen gets the science right. Pandemics are often caused by viruses that infect humans through zoonosis, hopping the species barrier from non-human to human hosts. Measles, smallpox, influenza, diphtheria all came from animals, with the most famous of course being HIV from chimpanzees. And this problem of pathogens ”spilling over” into humans becomes worse as we press into new habitats and animals face extinction pressure.

As an example, Quammen walks through how SARS migrated from bats to civet cats to humans. The SARS epidemic infected roughly 8000 people with roughly 800 deaths worldwide. It could have been worse, but epidemiologists reacted quickly. And SARS tended to make people very sick before they became contagious, so in total numbers the outbreak was scary but not too big.

One of the most telling and interesting parts of the book is when Quammen takes apart Richard Preston’s book The Hot Zone. The Hot Zone exaggerated the risks of Ebola, and made it seem like anyone with Ebola died hemorrhaging blood everywhere. In fact, most people with Ebola die of immune failure with no bleeding. As Preston himself conceded in his review of Spillover. But that didn’t stop the movie Outbreak from popularizing the explosive blood everywhere idea. And from there we’ve seen some great horror flicks about monkey viruses gone mad, like 28 Days Later or 12 Monkeys. Don’t get me wrong, I love those horror flicks. But Quammen was way too polite to Preston. To his credit, Quammen is a genuinely sympathetic writer who simply doesn’t like to trash people. But I also think Quammen is a fellow traveler. So even though Quammen gets his science right, he doesn’t mind an alarmist spin and goes easy on Preston’s far worse book.

A historical perspective here will help. Quammen is exactly right on the causes of pandemics. But in some sense his book is 700 years too late. Historically we could define “Peak Pandemic” as the years 1300-1600. During that time humans were doing what they do today, expanding into new habitats to devastating effect, getting up close and personal with all sorts of new animals and their viruses, trotting around the globe and causing extinctions. But the “golden age” of pandemics aligns to the start of the Age of Exploration, where we have the really big global push into virgin continents. As a consequence the pandemics from that era were far worse. Let’s look at some numbers. The Black Death from 1340-1400 killed roughly 30-60% of the population of Europe. That’s crazy high numbers. And the worst pandemic of all time was when Europeans came to the Americas in the 1490’s, bringing their diseases with them. This killed 90% of the indigenous American people. It emptied the Americas. Modern pandemics like SARS with 800 deaths just can’t compare, especially with modern medicine. What’s happening today is a mere shadow of the peak pandemic centuries. Today only the very last remnants of once flourishing ecosystems are being destroyed, and the death toll has been reduced accordingly.

The genetics of disease resistance are worth discussing here. We can think of resistance to disease as an arms race. As a population gets exposed to more and more diseases, a darwinian ratchet effect occurs, and only those with stronger immune systems survive. This comes at a cost of burning more of your energy on your immune system, but that’s a trade-off worth paying in a disease filled environment. Anyone alive today is a descendant of survivors of the great era of pandemics, and as such has a boosted immune system compared to the general population from 1000 years ago. The horrifying 90% death rate of indigenous americans is an unintended experimental proof. For a relatively recent (2006) account of disease in the Americas, see 1491: New Revelations of the Americas Before Columbus.

Another aspect of disease genetics is that it is not a good strategy for viruses to kill off their hosts too quickly. If they do, they can’t spread. Depending on how a virus is transmitted, it will evolve toward an optimal virulence. Something like HIV which bides it’s time has far more chance to spread.

And that’s really the bottom line here. HIV is exactly the type of pandemic we should expect more of in the modern world. The crazy monkeys vomiting blood pandemic from the movies is highly unlikely, though scientifically possible. So epidemiologists and doctors should keep an eye out for the next SARS or HIV. That’s an important job. But from a regular person point of view, we should just get our flu shots and worry more about getting hit by lightning. “Peak Pandemic” is hundreds of years in our past.

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