THE RULES OF CONTAGION
Why Things Spread — and Why They Stop
By Adam Kucharski

By now, most readers have surely heard of the science of disease modeling or the “R-naught,” the mathematical concept that expresses the number of new infections that a typical infectious person generates.

If the R-naught, or the basic reproductive number, is above one, then every person with Covid-19 will, on average, infect one or more people, and the prospect of being released from our pandemic purgatory diminishes. By contrast, if that number is below one, then, like a skier who has reached the top of a cable lift and whose only way out is down, we can soon look forward to resuming our pre-Covid lives.

Since SARS-CoV-2 emerged in or near an animal market in Wuhan, China, in the closing months of 2019, we’ve been mesmerized by this number and the modelers, such as Neil Ferguson at Imperial College, London, whose ability to plot the shifts of the R-naught have made them the secular sages of the pandemic.

Among their number is Adam Kucharski, an associate professor at the London School of Hygiene & Tropical Medicine who sits on a key scientific committee advising the British government on its response to the pandemic. For Kucharski the R-naught holds the key to what he calls the “rules of contagion” — rules that he asserts apply not only to biological infections but to social contagions and stuff that spreads online (think financial panics, gun crime and ice-bucket challenges).

But do they? Is contamination with a living virus — which presupposes the messy materiality of actual bodies — the same as a “virtual infection” with a piece of aberrant malware, or a catchy idea propagated via social media?

Up to a point. In this smart and engaging tour of epidemiology, written before the pandemic, Kucharski makes a convincing case that just as the arc of an epidemic depends on the transmissibility of a virus and a population’s susceptibility to infection, so online contagions obey similar rules. It all goes back to his hero, the early-20th-century British malariologist Ronald Ross, and his theory of “independent” and “dependent happenings.” The first type includes noninfectious diseases, such as brain cancer, accidents or divorce. If it happens to you, it generally doesn’t affect anyone else.

However, with dependent happenings, what happens to one person is a result of what has happened to others. In the simplest type of outbreak, affected people pass along the condition and, once affected, remain so. In this situation the happening will gradually permeate through the population and eventually plateau. Such epidemics follow the shape of a letter S, growing exponentially at first and then leveling off. By contrast, many infectious disease epidemics tend to look like inverted V’s because once people have been infected and have recovered, they are immune to further infection and the pool of susceptible people shrinks.

But whereas with contagious disease if you have good data you can estimate just how many people have been infected and how many are still susceptible — and hence the R-naught — this is not the case with financial contagions, where those affected can always borrow more money to leverage further trades. Nor is it true of conspiracy theories and other idiotic ideas that circulate online, for which there appears to be no end to people’s gullibility or susceptibility. (Worst of all, Kucharski says, is the gullible celebrity, who is both susceptible and hugely influential.)

The good news is that some social media platforms are finally recognizing they have a duty to inoculate users against such ignorance by slapping prominent warnings on erroneous and incendiary posts — or in the extreme cases, removing them. When it comes to Covid conspiracy theories, such as the daft idea that the virus is spread by 5G cell towers, users can also be directed to independent fact-checking sites, such as Full Fact.

“Along the way,” Kucharski writes, “we’re finding new ways to speed up beneficial ideas and slow down harmful ones.” Unfortunately, as with the search for a vaccine against Covid, that is easier said than done.