Book of the week
The Rules of Contagion: Why Things Spread – and Why They Stop by Adam Kucharski (Wellcome, $37)
How many of us are going to contract coronavirus? The answer – or, at least, the best means we have of trying to calculate it – started taking shape in the 1950s, when a researcher at the London School of Hygiene and Tropical Medicine wrote a paper on the control of mosquitoes. Hidden in the appendix was a novel idea about disease transmission: if you modelled what would happen when a single infectious person arrived in a population, it might give you a way to predict how serious an outbreak would be.
Twenty years later, a mathematician called Klaus Dietz picked up on the idea. What if you established the average number of people that a new case would infect? Call it the reproduction number, or R. You could then calculate how fast the disease would spread, how many people would catch it and, if you could intervene in a way that reduces R, how effective measures to fight it might be.
Suddenly, the R number looks less like an academic device and more like a matter of life and death. Coronavirus rates somewhere between R1.5 and R3.5, on current estimates. That sounds like a small range, but as Adam Kucharski’s book points out, it has huge consequences.
If the true value for coronavirus is found to be R2, that means every carrier infects two people; they each infect two more; and so on. After five rounds of infection, you have 32 new cases. Bad enough. Make that number R3, though – so every carrier infects three people – and after those same five rounds of infection, you have 243 new cases.
This is a hell of a moment for a book like this to come out, and some might assume it’s a lightningfast cash-in on a global tragedy. They would be wrong. Coronavirus hadn’t appeared when Kucharski delivered the manuscript, so the disease isn’t directly addressed here. But the principles of contagion, which, he argues, can be applied to everything from folk stories and financial crises to itching and loneliness, are suddenly of pressing interest to all of us.
Kucharski, a mathematician who has worked on the fight against the Ebola and zika viruses, believes that to master the diseases we first have to master the arithmetic. Vaccination, our most familiar weapon against epidemics, is so effective because it reduces the susceptible population: give just half of them immunity, and you halve the number each carrier can infect. Quarantine works in a similar way on the number of interactions carriers can have. Both can bring down that R number: get it below R1, and simple maths dictates that new cases will decline and the outbreak will end.
Kucharski’s maths is illuminating in other ways. Take schools: it turns out that, as parents have long suspected, they are a disease sink. In July 2009, H1N1 flu infections skyrocketed in the UK, but as soon as the school summer holidays ensured the kids were coughing over their PS4s rather than each other, the rate plunged to near zero.
So, will this book help you stay coronavirus-free? Maybe not in the way you might hope. Every outbreak is different – as Kucharski quips: ‘‘If you’ve seen one pandemic, you’ve seen... one pandemic’’ – so there’s only so much we can learn from previous contagions. The only sure way to dodge the bug is to reduce your personal interactions to zero. Stay home alone reading this for long enough, and you’ll be fine. Although the way coronavirus is shaping up, you might want to look out War and Peace, too.