INTRODUCTION It may not feel like it if you check the news, but you are living in a world that is far safer than the one in which your great--great--grandparents lived. Countless everyday experiences that used to pose terrible threats to our lives now pose no meaningful health risk at all. That glass of milk you had with dinner last night? A little more than a century ago, it might have contained germs that could cause the deadly disease tuberculosis. The scrape on your knee from falling off your longboard? That might have triggered a fatal infection just eighty years ago. Driving a car is ten times safer than it was when people first got behind the wheel. There are a thousand other improvements in health and safety like these, but they all add up to one amazing statistic, maybe the most important statistic of all. It''s what we call life expectancy : the number of years that the average person born at a given place and time can expect to live. Take a look at this chart of life expectancy in the United Kingdom, where they have been measuring it the longest: The average person born in the United States a century ago could expect to live a little more than forty years.

Today that number is just below eighty years. And Americans are four times more likely to live into their hundreds than they were a few decades ago. It''s an incredible transformation, and it''s not just happening in wealthy countries like the United States or the United Kingdom. A hundred years ago, life expectancy in India was only twenty--five years. Now it''s seventy years. As a species, it''s as though we''ve been given an entire extra life to live, compared to our ancestors. Some of that extra life is the result of elderly people living longer. Think about your own extended family.

Many of us have grandparents or great--grandparents who are in their eighties or nineties. Living that long is normal now. But it didn''t used to be. Another major factor in the story of life expectancy is how it changed what it means to be a child. For most of human history, about a third of all children died before they turned eighteen. Today it''s closer to one in a hundred. So why isn''t the story of extended life something we hear about all the time? Why isn''t it front page news? Because, most of the time, it doesn''t involve the sudden, dramatic changes that draw media attention. The changes that made our world so much safer were subtle, incremental ones.

They were slow and steady. In the long run, they add up to the most momentous transformation you could imagine. But in the short run, they are often invisible. One of the reasons we have a hard time recognizing this kind of progress is that, by definition, it is measured in nonevents: the smallpox infection that didn''t kill you at age two, the lead paint that didn''t give you brain damage, the drinking water that didn''t poison you with cholera. We don''t tend to think about these kinds of things for a good reason: they didn''t happen! But they might well have happened if you or I had been born just two centuries ago, even in the wealthiest countries in the world. Think about it this way: We build monuments and pay tribute to the lives lost in wars or great tragedies, and that makes sense. But we don''t have a habit of building monuments that celebrate deaths that didn''t happen, monuments to all the lives saved. Maybe it''s time we did.

In a sense, human beings have been increasingly protected by an invisible shield, one that has been built, piece by piece, over the last few centuries, keeping us ever safer and further from death. It protects us through countless interventions, big and small: the chlorine in our drinking water, the vaccinations that rid the world of smallpox, the data centers mapping disease outbreaks all around the planet. A crisis like the global pandemic that began in 2020 gives us a new perspective on all that progress. Pandemics have an interesting tendency to make that invisible shield suddenly and briefly visible. For once, we''re reminded of how dependent everyday life is on medical science, hospitals, public health authorities, drug supply chains, and more. And an event like the COVID--19 crisis does something else as well: it helps us perceive the holes in that shield, the vulnerabilities, the places where we need new scientific breakthroughs, new systems, new ways of protecting ourselves from emergent threats. Most history books focus on a central topic: a person, event, or place, a great leader, a military conflict, a city, or a nation. This book, by contrast, tells the story of a number: the rising life expectancy of the world''s population, giving us an entire extra life in merely one century.

It should help you understand where that progress came from, the breakthroughs and collaborations and institutions that had to be invented to make it possible. Some of the heroes of this story were scientists or doctors, but not all. Some of them were ordinary people who fought to improve the health of our species by writing articles or organizing protests or sharing an important breakthrough they had discovered in another culture. But they were all people who had a vision of how the world could be made a better place and who had the daring and the commitment to turn that vision into a reality. This book tells the story of how they pulled it off.