Twenty-five years ago, I published my first book, The Tipping Point: How Little Things Make a Big Difference. Back then I had a little apartment in the Chelsea neighbourhood of Manhattan, and I would sit at my desk, with a glimpse of the Hudson River off in the distance, and write in the mornings before I headed to work. Because I had never written a book, I had no clear idea how to do it. I wrote with that mix of self-doubt and euphoria common to every first-time author.
“The Tipping Point is the biography of an idea,” I began, “and the idea is very simple. It is that the best way to understand the emergence of fashion trends, the ebb and flow of crime waves, or, for that matter, the transformation of unknown books into bestsellers, or the rise of teenage smoking, or the phenomena of word of mouth, or any number of the other mysterious changes that mark everyday life, is to think of them as epidemics. Ideas and products and messages and behaviours spread just like viruses do.”
The Tipping Point was published in the spring of 2000. The first stop on my book tour was a reading at a small independent bookstore in Los Angeles, to which two people came, a stranger and the mother of a friend of mine – but not my friend. (I have forgiven her.) I said to myself, Well, I guess that’s it. But it wasn’t! The Tipping Point grew like the epidemics it described – at first gradually, then all in a rush. By the time the paperback came out, it had entered the zeitgeist.
Do I know why The Tipping Point touched such a chord 25 years ago? Not really. But if I had to guess, I would say that it was because it was a hopeful book that matched the mood of a hopeful time. The new millennium had arrived. Crime and social problems were in freefall. The cold war was over. I offered in my book a recipe for how to promote positive change.
Twenty-five years is a long time. And so I thought it might be interesting to revisit The Tipping Point to reexamine what I wrote so long ago. But as I immersed myself once again in social epidemics, the world seemed very different to my eyes. I had not reread The Tipping Point in the years since its publication, and when I finally did, I stopped every few pages to ask: What about this? How could I have left out that?
Twenty-five years ago, I argued that the laws of epidemics could be used to promote positive change: lower crime rates, teach kids how to read, curb cigarette smoking. Now I wanted to look at the underside of the possibilities I explored so long ago. If the world can be moved by just the slightest push, then the person who knows where and when to push has real power. So who are those people? What are their intentions? What techniques are they using?
I’m not convinced that we fully appreciate the implications of the way epidemics operate – even after going through a prolonged and painful crash course on the subject during the Covid crisis.
Let me give you an example. Years ago, I went to see a remarkable man named Donald Stedman. (He died in 2016.) He was a chemist at the University of Denver, and a brilliant inventor. One of his many creations was an elaborate contraption that used infrared light to instantly measure and analyse the emissions of cars as they drove by on the highway. I flew to Denver, where Stedman had hooked up his invention to a big electronic sign. Whenever a car with properly functioning pollution control equipment passed, the sign flashed good. When a car passed that was over the acceptable emissions limit, the sign flashed poor.
We must have sat there, watching, for an hour. What quickly became apparent was that a poor rating was incredibly rare. Yet, Stedman said, those few cars were the primary cause of Denver’s air-pollution problem. For whatever reason – age, ill repair, deliberate tampering by the owner – a small number of automobiles were producing carbon-monoxide levels as much as 100 times higher than the average.
In Denver in 2006, Stedman discovered, 5% of the vehicles on the road produced 55% of the automobile pollution. That’s the Law of the Few: it’s a very large problem caused by a very small number of actors.
Stedman’s idea was that someone should set up his devices around Denver, and have a police officer pull over anyone who fails. A half dozen of his roadside smog checkers, Stedman estimated, could test 30,000 cars a day – which, in a few years, would translate to a reduction in emissions in the Denver area of 35 to 40%.
Since Stedman’s pioneering work, other researchers have conducted similar kinds of tests all over the world. And the results are always the same: somewhere around 10% of vehicles are, at any given time, responsible for over half the automobile-based air pollution. The distribution of polluting cars is – to borrow a phrase used in one study of drivers in Los Angeles – “extremely skewed”.
In another study, a group of Italian researchers calculated how much Rome’s air quality would improve if 10% of the city’s cars were electric-powered. As you would imagine, it would make a big difference. But then they did a second calculation: what would happen if the city required just the top 1% of polluters to go electric? Pollution would fall by the same amount. Nearly 40 years after Donald Stedman invented his magic contraption, almost everyone agrees with him. So what has happened in Denver since Stedman started putting up his roadside testing sites? Nothing. The state of Colorado still makes most drivers get a regular emissions check, and Denver’s air quality – which was pretty good in the 2000s – has gotten worse in the past decade.
Urban air pollution is a perfect example of a problem caused by the few. But we behave as if it’s a problem caused by all of us. No one wants to act on the asymmetry, and it’s not hard to understand why: singling out a handful of big-time polluters would make the job of the people who worry about Denver’s air quality a lot harder. What if the people pulled over were disproportionately poor? What if they couldn’t afford to get their cars fixed? Do you confiscate their cars if they don’t comply?
Moving from the position that a problem belongs to all of us to the position that a problem is being caused by a few of us is really difficult. And we are so intimidated by that difficulty, apparently, that we’d rather breathe dirty air. This is a problem that is very much in our future. Technology is going to give us the ability to figure out who the special few are – not just on roadsides in Denver but in all kinds of places, including at the outset of a pandemic. What will we do with that information?
In the early 1970s, there was a measles outbreak at an elementary school outside Rochester, New York. Because 60 children fell ill, local health officials felt compelled to launch an investigation. They collected medical records, analysed maps of the school, calculated how the ventilation system worked, figured out who rode the bus home and who didn’t, and where every infected child sat in their classroom. From that, they were able to reconstruct the path of the virus. The outbreak, they learned, came in two waves. Twenty-eight students got infected in the first wave, who eventually passed on the virus to another 31 kids.
But then they stumbled onto something strange. It had to do with how the first wave of 28 schoolchildren got sick. It was from one person: a girl in second grade. And her case made no sense. She didn’t ride the bus to school, which the investigators thought was one of the likeliest places for transmission to happen. Nor did she infect students just in her own classroom, which is also the likely scenario for the spread of an infectious virus. Instead, she infected children across 14 different classrooms. In the models that epidemiologists used to understand the spread of diseases such as measles, the assumption was that every infected person had roughly the same chance of passing on their virus to someone else. But this little girl made a mockery of that assumption. The only way to make sense of that inexplicable first wave was if she exhaled 10 times more virus particles than the typical measles patient.
“We are intrigued by the possibility of an order of magnitude difference between the infectiousness of the index case and the subsequent cases,” the investigators wrote.
Intrigued, it’s safe to say, was an understatement. It took a long time for this idea – that some people might excel at infecting others – to take hold in the scientific world. For years, there were scattered reports in the medical literature, the epidemiological equivalent of UFO sightings. But no one knew quite what to do with cases like this. They didn’t fit easily into the existing models about how epidemics work.
The term super-spreader didn’t come into regular use until the end of the 1970s, but even then the concept remained theoretical. There were too many unanswered questions. Everyone understood that, say, a 6ft 5in man weighing 275lb would pose more of a threat in spreading a respiratory virus than a 100lb woman. His lungs were much bigger! But height and weight alone could not explain away the fact that a second grader was exhaling 10 times more measles particles than her classmates.
The doctors in Rochester were flummoxed. They knew who their super-spreader was, yet they couldn’t figure out what made her any different.
Enter the aerosolists. Aerosolists are scientists whose job it is to understand the properties and behaviour of tiny airborne particle – aerosols. What’s actually in the smoke that comes out of a chimney, or the smell from cooking bacon? Those are the kinds of things that aerosolists think about.
One of the most important tools in the aerosol world is an aerodynamic particle sizer, or APS machine. It’s a box, fed by a funnel. It’s the human equivalent of the magic box that Stedman invented for measuring the emissions of cars. If you breathe into it, it runs the air that comes out of your mouth through a series of lasers, which count the number and measure the size of every aerosol particle in your breath. So William Ristenpart’s lab gathered 48 volunteers and had them breathe into an APS. The study subjects repeated vowel sounds. They raised and lowered their voices. They performed “vocalisations”. And the researchers confirmed what all those UFO sightings over the years had suggested: a small group of their sample was off the charts.
“That’s what we call superemitters,” Ristenpart said. “Some people just released about an order of magnitude more aerosols for the … same given observed loudness.” He went on, “We had no idea. If I had to go back to the beginning, I probably would’ve hypothesised: different people have different-sized distributions. But I didn’t guess it would be an order‑of‑magnitude difference between people.”
Another leading aerosolist, Harvard’s David Edwards, found the same pattern. He didn’t focus on talking. He travelled to Asheville, North Carolina and Grand Rapids, Michigan and measured the breathing of a group in each city. He ended up testing 194 people. The overwhelming majority were low spreaders: they would be hard-pressed to infect anyone. But there were 34 whom Edwards called high producers. Of those, 18 were super-high spreaders, and within that elite group of super-high spreaders was one person who exhaled, on average, an astonishing 3,545 particles per litre – over 20 times more than the largest group of low spreaders.
Finally, near the end of the pandemic, came the decisive bit of evidence. As part of a “challenge study”, British researchers deliberately infected 36 willing volunteers with Covid. All of them were young and healthy. They were all exposed to the exact same dose of the exact same strain at exactly the same time under exactly the same conditions. All were then quarantined in a hospital, enabling the researchers to put them under a medical microscope, monitoring and testing every symptom and vital sign. And what did they find? A full 86% of all of the Covid virus particles detected in their group of infected volunteers came from … two people. Airborne viruses do not operate according to the Law of the Few. They operate according to the Law of the Very, Very, Very Few.
“For unclear reasons, certain individuals are ‘speech superemitters’ who emit an order‑of‑magnitude more aerosol particles than average,” Ristenpart and his colleagues wrote in their Aerosol Science and Technology manifesto. In other words, a certain kind of individual – like that little girl in Rochester— produces lots of aerosol particles as part of their genetic makeup. Ristenpart thinks that super-spreaders might be people who, by some quirk, have saliva with unusual properties: their saliva is more elastic and more viscous – thicker and stickier – than normal. So when they break through those liquid bridges across their vocal cords, more aerosols are produced.
David Edwards, for his part, believes that whatever individual differences exist might be amplified by something as simple as hydration. “Your upper airways are like a car wash,” he says, “and the air that comes into your upper airways is like a car.” When the car wash is working properly, the vast majority of the little bits and pieces in the air you breathe get washed away. “If you stay well hydrated, your upper airways will capture pathogens all the time, and they move them – within 20 minutes or an hour – out into your gut and you swallow … and they’re eliminated that way,” Edwards said. “But when you’re dehydrated, there’s no water in the car wash.” That’s why being dehydrated makes you more vulnerable to colds and the flu and Covid: when you exhale, those virus particles come back out – and now you are more likely not just to contract a virus but to spread it. The particles hit your dry airways and break up into a concentrated, foamy spray, like a big wave hitting a beach. That’s how you get to 3,545 particles per litre. So, which people tend to have dehydrated upper airways?
When Edwards looked at his breathing data, he found that the biggest predictors of high production of aerosols were age and body-mass index (BMI).
We don’t yet know which – if any – of these explanations is correct. But it seems certain that one day scientists will know, and that discovery will create an industrial-size version of the dilemma we faced with Stedman’s roadside-emissions plan.
What if age and obesity really are the two biggest predictors of super-spreading? Does that mean in the middle of a pandemic passengers will refuse to sit beside an overweight person on a plane? What if the answer is viscous saliva, and a scientist comes up with a 10-second test to measure if someone is in the 99th percentile? Would a restaurant or a movie theatre or a church be justified in asking everyone to take a saliva test at the door?
Stedman would have said, in answer to his detractors, that all of these objections are well and good, but at a certain point the city of Denver has to decide how serious it is about cleaning up its air. This will be true of the next deadly virus as well. We will have to decide how far we are willing to go in order to save lives.