I recently read Spillover: Animal Infections and the Next Human Pandemic, by David Quammen. This book was written eight years ago, but it helped me understand what’s happening today with the coronavirus. Quammen writes:
A person might construe this list [Ebola, HIV, bird flu, West Nile virus, SARS, and now Covid-19] as a sequence of dire but unrelated events-independent misfortunes that have happened to us, to humans, for one unfathomable reason and another. Seen that way,Machupo and the HIVs and SARS and the others are “acts of God” in the figurative (or literal) sense, grievous mishaps of a kind with earthquakes and volcanic eruptions and meteor impacts, which can be lamented and ameliorated but not avoided. That’s a passive, almost stoical way of viewing them. It’s also the wrong way.
Make no mistake, they are connected, these disease outbreaks coming one after another. And they are not simply happening to us; they represent the unintended results of the things we are doing. They reflect the convergence of two forms of crisis on our planet. The first crisis is ecological, the second medical. As the two intersect, their joint consequences appear as a pattern of weird and terrible new diseases, emerging from unexpected sources and raising deep concern, deep foreboding, among the scientists who study them. How do such diseases leap from nonhuman animals into people, and why do they seem to be leaping more frequently in recent years? To put the matter in its starkest form: Human-caused ecological pressures and disruptions are bringing animal pathogens ever more into contact with human populations, while human technology and behavior are spreading those pathogens ever more widely and quickly.
Spillover doesn’t contain much physics, but it does allude to the math describing epidemics, making it relevant to Intermediate Physics for Medicine and Biology. Chapter 3 of Spillover discusses the 1927 model of William Kermack and Anderson McKendrick. I admire Quammen for including the mathematical biology of epidemics in his book, but he seems uncomfortable talking about math, and hesitant about subjecting his readers to it. I’m glad the readers ofIPMB don’t place me under the same constraint.
Amid a dense flurry of mathematical manipulations, [Kermack and McKendrick] derived a set of three differential equations describing the three classes of living individuals: the susceptible, the infected, and the recovered. During an epidemic, one class flows into another in a simple schema, S → I → R, with mortalities falling out of the picture because they no longer belong to the population dynamic. As susceptible individuals become exposed to the disease and infected, as infected individuals either recover (now with immunity) or disappear, the numerical size of each class changes at each moment in time. That’s why Kermack and McKendrick used differential calculus. Although I should have paid better attention to the stuff in high school [I didn’t take calculus until college!], even I can understand (and so can you) that dR/dt = γI merely means that the number of recovered individuals in the population [I would have said “the rate of increase of the number of recovered individuals…”], at a given moment, reflects the number of infected individuals times the average recovery rate. So much for R, the “recovered” class. The equations for S (“susceptibles”) and I (“infected”) are likewise opaque [not the word I would choose] but sensible. All this became known as an SIR model. It was a handy tool for thinking about infectious outbreaks, still widely used by disease theorists.
Covid-19 is caused by a zoonotic virus: a pathogen that leaps from an animal “reservoir” to infect humans. Quammen focuses on zoonotic viruses in Spillover, but he points out that not all viruses originate in animals. For instance, polio and smallpox are viruses that infect only humans. Once we remove those viruses from the human population, they are gone forever. A zoonotic virus “hides” in some wide animal ( bats are a common reservoir) until it makes the jump to humans, so they are extraordinarily difficult to eradicate. Spillover is at its best when it describes these jumps, and the scientists who study them. Moreover, Quammen’s book is an extended case study of the scientific method. Everyone should read it.
Does Quammen predict the Covid-19 pandemic? Sort of. He predicts future pandemics arising from virulent and transmissible viruses that spill over from animal reservoirs. He predicts that our growing population and technology will make such spillovers more common. He even pinpointed coronaviruses as one of the likely suspects that could cause a future plague. What scares me is that Covid-19-as disruptive as it’s been for society-is not virulent enough to be the “Next Big One.” I fear it may be only a hint of things to come.
Worried? Me too. I’ll let Quammen have the final-somewhat hopeful-word [my italics].
I don’t say these things about the ineradicability of zoonoses to render you hopeless and depressed. Nor am I trying to be scary for the sake of scariness. The purpose of this book is not to make you more worried. The purpose of this book is to make you more smart.