The Rules of Contagion: Why Things Spread - and Why They Stop

by Adam Kucharski

Although his ideas eventually started to catch on, he lamented the delay. ‘The world requires at least ten years to understand a new idea,’ he once wrote, ‘however important or simple it may be.’

When there are enough immune people to prevent transmission, we say that the population has acquired ‘herd immunity’.

Likewise, herd immunity meant that the population as a whole could block transmission, even if some individuals were still susceptible.

According to the SIR model, outbreaks need three things to take off: a sufficiently infectious pathogen, plenty of interactions between different people, and enough of the population who are susceptible. Near the critical herd immunity threshold, a small change in one of these factors can be the difference between a handful of cases and a major epidemic.

In large epidemics, transmission generally slows down because there aren’t many susceptible people left to infect. For the epidemic to keep increasing faster and faster, infectious people would have to actively start seeking out the remaining susceptibles in the later stages of the epidemic. It’s the equivalent of you catching a cold, finding all your friends who hadn’t got it yet and deliberately coughing on them until they got infected. The most familiar scenario that would create this outbreak shape is therefore a fictional one: a group of zombies hunting down the last few surviving humans.

If R is 10 in a fully susceptible population, we’d need to vaccinate at least 9 in every 10 people. If R is 20, as it can be for measles, we need to vaccinate 19 out of every 20, or over 95 per cent of the population, to stop outbreaks. This percentage is commonly known as the ‘herd immunity threshold’.

‘As they say in poker, if you don’t know how to spot the sucker at the table, it is you.’

As physicist Max Planck supposedly once said, ‘science advances one funeral at a time.’

There can be a similar effect with mass shootings; one study estimated that for every ten US mass shootings, there are two additional shootings as a result of social contagion.

But easy access to deadly methods can make a difference for what are often impulse decisions. In 1998, the UK switched from selling paracetamol in bottles to blister packs containing up to thirty-two tablets. The extra effort involved with blister packs seemed to deter people; in the decade after the packs were introduced, there was about a 40 per cent reduction in deaths from paracetamol overdoses.

For a riot to start, there need to be at least some people willing to join. ‘You cannot riot on your own,’ as crime researcher John Pitts put it. ‘A one-man riot is a tantrum.’

One of the most vocal online communities is the anti-vaccination movement. Members often congregate around the popular, but baseless, claim that the measles-mumps-rubella (MMR) vaccine causes autism. The rumours started in 1998 with a scientific paper – since discredited and retracted – led by Andrew Wakefield, who was later struck off the UK medical register. Unfortunately, the British media picked up on Wakefield’s claims and amplified them.

Metrics have even shaped literature. When Alexandre Dumas first wrote The Three Musketeers in serialised form, his publisher paid him by the line. Dumas therefore added the servant character Grimaud, who spoke in short sentences, to stretch out the text (then killed him off when the publisher said that short lines didn’t count).[75]

Spoiler alert!

Jonathan Albright spent early 2017 visiting over a hundred extreme propaganda websites, the sort of places that are full of conspiracy theories, pseudoscience, and far-right political views. Most of the websites looked incredibly amateurish, the sort of thing a beginner would put together. But digging behind the scenes, Albright found that they concealed extremely sophisticated tracking tools. The websites were collecting detailed data on personal identity, browsing behaviour, even mouse movements. That allowed them to follow susceptible users, feeding them even more extreme content. It wasn’t what users could see that made these websites so influential; it was the data harvesting that they couldn’t.

On my first trip to mainland China, I remember trying to connect to WiFi when I arrived at my hotel. It took me a while to work out whether I was actually online. All the apps I usually might load to check my connection – Google, WhatsApp, Instagram, Twitter, Facebook, Gmail – were blocked. As well as demonstrating the power of the Chinese firewall, it made me realise how much influence US technology firms have. The bulk of my online activity is in the hands of just three companies.

In the time it’s taken you to read this book, around three hundred people will have died of malaria. There will have been over five hundred deaths from hiv/aids, and about eighty from measles, most of them children.