Did 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.

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.

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 before that particular bubble burst in 1720, and used his profits to found Guy’s Hospital in London. Isaac Newton wasn’t so lucky. “I can calculate the motion of heavenly bodies but not the madness of people,” he allegedly said, after losing the equivalent of £20m in today’s money.

One of the most interesting and topical parts of the book is about the mathematical modelling of fake news. We already knew this was contagious – albeit with its own specific mechanisms of contagion – but as we are seeing yet again, a pandemic of fake news can feed off, and interact with, a pandemic of infectious disease. Happily, the modellers are beginning to get a handle on how fake news spreads, and how to stop it, and their insights are already being translated into practice. In the Democratic Republic of the Congo, which now has Covid-19, polio and measles as well as Ebola, a third of the workforce involved in the Ebola response are social scientists whose job is to build bridges with local people, understand their thinking around the disease, and reinforce sound information while countering rumours.

Memory, too, is contagious – and so is forgetting. We collectively forgot the worst catastrophe of the 20th century, the so-called “Spanish” flu that happened just over a century ago, and now it’s all anyone can talk about. Will that revived memory outlast the new pandemic, or will it sink back into the sediment of our minds as soon as the danger has passed, and the social networks have moved on? It’s too early to say, but one could argue that our long forgetting of that pandemic weakened our incentive for investing in healthcare systems and doing all the other things that public health experts have been urging us to do for decades, to prepare ourselves for the next one. A contagion of forgetting prompted a pandemic which prompted a contagion of remembering.

Modellers have a saying: “If you’ve seen one pandemic, you’ve seen … one pandemic.” Covid-19 doesn’t behave like flu, which doesn’t behave like Ebola. The title of The Rules of Contagion refers to the things that are common to all pandemics – from the ice bucket challenge to bitcoin to infectious diseases – rather than those that are peculiar to each one. Modellers have spent the last century teasing out those rules, and now their conclusions are being put to the mother of all tests. This book explains how they arrived at them, and why they indicate that what each of us does next will make a difference. There may still be a lot of uncertainty around Covid-19, but one thing is clear: for Kucharski and his fellow modellers, it will be one more learning experience.

• Laura Spinney’s Pale Rider: The Spanish Flu of 1918 and How It Changed the World is published by Vintage. The Rules of Contagion is published by Profile (RRP £16.99). To order a copy go to guardianbookshop.com. Free UK p&p over £15.