The Rules of Contagion by Adam Kucharski review – outbreaks of all kinds | Books

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Modellers have a saying: “If you’ve seen one pandemic, you’ve seen … one pandemic.” But patterns can be established to do with how things spread. D id you notice? There was a moment when something shifted, and all topics of conversation besides Covid-19 started to sound trivial. Things will surely shift again, as people realise that the self-confinement could last and escapism becomes our collective goal, but for now Adam Kucharski’s The Rules of Contagion is the book you might want to reach for. Not least – given that the present pandemic is very much in the ascendant – for its subtitle: Why Things Spread – and Why They Stop.
Kucharski, an epidemiologist at the London School of Hygiene and Tropical Medicine, tells the story of the mathematical modelling of infectious disease, about which we have heard so much lately. The book’s hero is Ronald Ross , the British doctor who in the late 19th century discovered that mosquitoes spread malaria and was rewarded with the 1902 Nobel prize in physiology or medicine.
Ross wasn’t the first to describe an epidemic mathematically, but he was the first to do so armed with a thorough understanding of the biological and social processes that shape it, and this made all the difference. Whereas others had looked backwards, observing how epidemics unfolded over time, he looked forwards, making predictions about the course of an outbreak and how various interventions might affect it. His ideas form the basis of modern disease modelling, and of our governments’ strategies to combat Covid-19. It’s partly thanks to Ross that we have the concept of herd immunity – hopeful because it means that not every mosquito has to be squashed, not every person has to be vaccinated, for a population to be protected against a disease.
Facebook Twitter Pinterest A Red Cross worker pours a sanitising gel into a driver’s hand in Dakar, Senegal. Photograph: Zohra Bensemra/Reuters
Epidemic is a Greek word meaning “on the people”, and until Hippocrates requisitioned it to refer exclusively to the spread of a disease, the Greeks applied it to anything that percolated through a population – from fog to rumour to civil war. What’s striking about Kucharski’s tale is how we’ve circled back to that pre-Hippocratic outlook. Ross and mathematician Hilda Hudson set the ball rolling when they folded their ideas about the spread of disease into a broader “theory of happenings”, but they distinguished between “dependent” happenings, such as a contagious disease, and “independent” happenings, which could not be passed from person to person. In the latter category they placed accidents, divorce and chronic diseases – the kind that kill most of us today.
A century on, ideas have changed. Now the thinking is that many of the things that Hudson and Ross might have considered independent – obesity, smoking, even loneliness – are catching, too. We talk about financial contagion and epidemics of knife crime, and methods borrowed from public health are being applied to try to nip these problems in the bud, or at least slow their spread. Remember when Glasgow had the ignominious reputation of being the murder capital of Europe, known for its signature “Glasgow smile”? Then the Violence Reduction Unit was set up there in 2005, modelled on similar projects in the US, and the city’s homicide rate was cut by two-thirds.
Ross would be surprised, gratified and perhaps also slightly amused. The man who explained that a disease can only get a foothold in a population if new infections outnumber recoveries complained in his lifetime about how long it took for new thinking – his – to catch on. “The world requires at least 10 years to understand a new idea,” he wrote, “however important or simple it may be.” In the 21st century, everyone has got with the programme, including marketers whose goal is to ramp up the transmissibility of an idea or meme as far as they possibly can. Just like the Covid-19 modellers, they build whole strategies around the reproduction number R – the classic measure of the transmissibility of a happening.
Facebook Twitter Pinterest The trading floor of the New York Stock Exchange after workers tested positive for Covid-19 on 19 March. Photograph: Lucas Jackson/Reuters
It’s notable, too, that Kucharski’s protagonists flit easily between the very disparate worlds in which these models are now routinely applied. Marine ecologist George Sugihara was wooed away from his study of fish populations to build predictive models for Deutsche Bank. “Basically,” he observed later, “I modelled the fear and greed of mobs that trade.” Epidemiologist Gary Slutkin moved from public health in Africa to crime reduction in Chicago, and Kucharski himself interned at an investment bank in Canary Wharf before turning his attention to the spread of dengue fever and Zika virus disease in the Pacific. And on the theme of crossovers, he includes the pleasing anecdote that bookseller Thomas Guy got out of the South Sea Company just befo...