The Rules of Contagion Why Things Spread – and Why They Stop

With the world in the throes of the COVID-19 pandemic, the questions posed by the subtitle of this book are on everyone's mind. Associate Professor at the London School of Hygiene & Tropical Medicine Adam Kucharski here takes the reader through the inner workings of contagion. From violence and ideas to financial crises and, of course, disease – some universal rules cut right across disciplines. So, is this the most topical book of the year? Well, yes and no.

Kucharski is uniquely positioned to write a wide-ranging book like this. Next to his current position, he is trained in mathematics and did a spot of interning with a bank when the 2008 financial collapse hit. As such, he is at ease explaining both epidemiology and investment banking.

On the disease front, Kucharski covers recent outbreaks of fairly novel diseases such as Zika (which he encountered first-hand in Fiji in 2015), AIDS, Ebola, and SARS. He does not provide the full history of these the way David Quammen did in Spillover, but nevertheless gives you the relevant points in a concise form. Similarly, there is attention for some historical cases such as the miasma theory (the idea that bad air was behind diseases) and how John Snow's work on cholera in 1850s London disproved this (see also my review of Virusphere).

Particularly relevant to this moment in time are the epidemiological details, the titular rules of contagion. Kucharski introduces you to the SIR model, which describes how people move through three groups during a disease outbreak (Susceptible, Infectious, Recovered), how this plays into the concept of herd immunity, and how vaccination influences this. He explains the reproduction number R, its four components, abbreviated DOTS (R = Duration × Opportunities × Transmission Probability × Susceptibility), and how this explains why measures such as washing your hands and social distancing have an effect. And then there are those mysterious superspreaders which requires Kucharski to delve into network topology (the architecture of connections in a network). Having read this book, you should come away with a far better understanding of these parameters and mechanisms.

But here is the kicker of the book: these rules are not unique to disease outbreaks. Ideas from public health can and have been applied to numerous other fields. This sees Kucharski branch out widely and cover a huge number of seemingly unrelated topics. His internship with a bank has given him an insider's view of financial contagion, allowing him to clarify pyramid schemes and financial bubbles, but also how the notion of superspreaders applied to the 2008 banking crisis.

The transmission of ideas similarly follows many patterns seen in disease outbreak, although Kucharski is careful to consider ideas other than social contagion for how information spreads. His reminder of some of the psychological biases that can hinder or encourage the spread of information is a topic that will never lose its relevance. On the other side, online contagion can be encouraged by e.g. social media companies who apply epidemiological knowledge to viral marketing and the never-ending battle for your attention. This has a darker side too, ranging from echo chambers and online manipulation to the privacy concerns of many citizens about the amount and nature of data harvested by these companies.

Of course, you cannot talk about technology without touching on computer viruses, and Kucharski is equally capable of writing engagingly about computer viruses, malware, botnets, DDoS attacks, or the danger of poorly secured devices that are forming the Internet of Things. Or what of the little-known habit of programmers to borrow pieces of code for online applications that all need to be called on, creating a vulnerable chain of dependency? This chapter might at the outset feel like a digression. But Kucharski beautifully circles back to the topic at hand by showing the parallels between virus evolution in both living and artificial systems. It is a neat writing tactic that crops up several times.

More eye-opening for me was the long history of applying epidemiological ideas to public health. The spread of violence, riots, even suicide, can be studied and understood in this framework. One topic that made me squeal with delight was how phylogenetics (the study of evolutionary relatedness by identifying common ancestors) can be applied to a completely different field such as the history of folk tales.

From the above, it is clear that, next to disease outbreaks, The Rules of Contagion ricochets off a huge number of topics. Not all of these will interest everyone, but the enthusiasm with which Kucharski covers them is tangible, and the universal relevance of the epidemiological rules striking. Some of his metaphors are particularly lucid. Of models, he writes that they are "just a simplification of the world, designed to help us understand what might happen in a given situation [...] particularly useful for questions that we can't answer with experiments". While of the difficulty of applying phylogenetic analysis to a slowly evolving pathogen, such as measles, he writes it is "a bit like trying to piece together a human family tree in a country where everyone has the same surname".

The timing of publication of this book was uncanny, right as the COVID-19 pandemic started ramping up around the world. This presents both a blessing and a curse. On the one hand, there is a sudden, huge interest in the topic of epidemiology, and the publisher has understandably been keen to emphasize this while marketing The Rules of Contagion. On the other hand, its publication in March 2020 meant that the writing for it will have finished when the pandemic was but Chinese whispers on the wind.

Given the urgency with which people now want accessible information, many will come to this book with a narrow focus of interest and might end up frustrated or disappointed with what they see as too many irrelevant asides. Some Amazon reviews suggest this has already been the case. Personally, I think this is both unfair and misses the point. One only has to look at Kucharski's Twitter feed to see how involved he is with the ongoing pandemic. If he had finished writing it later, would it have been a different book? I would not be surprised if the paperback will contain a new introduction or post-script. Or Kucharski might follow Quammen's example. A year after Spillover was published he excerpted and adapted part of it as Ebola at the start of the 2014-2016 Ebola outbreak.

Whatever Kucharski will do next, here is a writer to keep an eye on. The Rules of Contagion is an incredibly engaging piece of cross-disciplinary popular science that will hold its relevance well beyond the COVID-19 pandemic.