Twenty-five years ago, I encountered a question that I have thought about literally every day since: Why do children die?

Before I tell you what drew me to this mystery, I want to acknowledge that child mortality is not an easy subject to talk about. As a parent, I can’t imagine what it would be like to lose a child. It is shocking even to see the words “children” and “die” used in the same sentence.

But I think “why do children die?” is one of the most important questions ever. It is hard to think of a measure of how a society is doing that reveals more than whether it is protecting its children, and especially its most vulnerable children. And the better we understand why children die, the more we can do to save them.

The very good news is that the world has made phenomenal progress in this area over the past several decades. Since 1990, the number of children who die every year has fallen by more than half! If progress on child mortality is a good measure of the state of the world, then—despite the huge global setbacks of the past few years, including COVID-19—the state of the world has improved dramatically. And based on what I know about innovations that are still to come, we can look forward to even more progress in the years ahead.

My introduction to the subject came 25 years ago, when I read a New York Times article about the health problems caused by unsafe drinking water in low- and middle-income countries. I was shocked to learn that every year, 3.1 million people—nearly all of them children—died of diarrhea, often because they had drunk contaminated water. Diarrhea kills 3.1 million children?, I thought. That can’t be true, can it? But it was.

I had to know more. What other major inequities did I not know about?

I read everything about global health that I could find, and I spoke to as many experts as I could. I learned that researchers define child mortality as the death of anyone under the age of 5. They use that age because the first five years are the riskiest time of childhood, when kids are the most vulnerable.

Learning about the history of child mortality helped me put the statistics in context. In 1950, some 20 million children died. In 1990, it was down to 12 million children, even though more babies were being born. By 2000, the number had dropped to fewer than 10 million. By 2019, it was below 5 million. Virtually all of these deaths occur in low- and middle-income countries.

So the next question was, why were so many children dying?

Around 18 percent of the deaths were caused by non-communicable conditions, such as cancer and cardiovascular problems. The large majority—82 percent—of the deaths were caused by communicable diseases, such as diarrhea and malaria, and health problems that their mothers experienced—and exacerbated by risk factors including malnutrition. (This 18:82 ratio still holds true today.)

On one hand, this was heartbreaking. The worst killers were all things that people in rich countries considered just an unpleasant episode (such as diarrhea) or never experienced at all anymore (such as malaria). In other words, although it was obviously true that children were dying because of deadly diseases, that was only part of the explanation. They were also dying because of where they were born.

On the other hand, it was encouraging to learn that such a large share of the deaths was preventable. When I saw the breakdown of diseases, I thought: Here is our road map. This is what the Gates Foundation should be working on. With the right team, partners, and funding, we could help the world move through the list, systematically going after the worst killers. The solutions that already existed could be made more affordable and delivered to people in low-income countries. The ones that didn’t exist could be invented.

Here is the chart as it looks today:

As you can see, pneumonia is the top preventable cause, but the story here is one of real progress. In 2000, it took the lives of more than 1.5 million children, but by 2019, the number was around 670,000—still an awful number, but a reduction of more than 55 percent. The innovation related to pneumonia that’s going on today is so exciting that I made a separate post and video about it.

Diarrhea is another example of progress. In two decades, its death toll has dropped 58 percent. A key reason is the use of low-tech interventions like oral rehydration solution (sugar water, essentially), which replaces lost electrolytes. Governments also ran large-scale sanitation programs to cut down on the spread of bacteria. And scientists developed an affordable rotavirus vaccine, and the world came together to deliver it. Between 2010 and 2020, this vaccine prevented more than 200,000 deaths. By 2030, it will have prevented more than half a million deaths.

Even though the overall number of deaths has gone down by half, the relative positions of the top three killers have not changed. They are the same today as in 1990: neonatal disorders, pneumonia, and diarrheal diseases. As you can see in this graphic, the fourth slot is where there has been a huge shift. In 1990, it was occupied by measles, responsible for half a million deaths. Today, it’s malaria that is in the fourth slot—not because malaria deaths went up (they actually went down), but because measles deaths fell by a whopping 87 percent.

Why? Vaccines. Since 2000, Gavi, the Vaccine Alliance has provided measles vaccines to more than 500 million children—half a billion!—through routine immunization and special vaccination campaigns. (This is just one example of the magic of vaccines—although unfortunately vaccination rates have dropped because of the pandemic and other factors.) And malaria may not be #4 on that list for long, thanks to innovations like malaria vaccines, improved insecticide-treated bed nets, and sugar baits.

Many groups deserve credit for the decades of progress I’ve described in this post. Countries with high disease burdens have launched massive vaccination campaigns, strengthened their health systems, and shared best practices with each other. Wealthy countries generously give aid that supports these efforts. Pharmaceutical companies have contributed technical expertise and made products affordable for low- and middle-income countries. Foundations including the Gates Foundation have stepped up with additional funding for innovative ideas. (At the foundation, we have staff and partners dedicated to each slice of the pie you see above.)

Although it’s still true that too many children do not live to see their fifth birthday, the world is moving in the right direction. If everyone keeps doing their part, we can move even faster and save even more lives. Because of COVID and other setbacks, the United Nations’ goal to cut childhood deaths in half again to below 3 million by 2030 will be missed, but it can still be achieved the following decade.

At a time when war and pandemic are in the news every day, it is important to look for reasons to be hopeful. The world’s opportunity—and ability—to save children’s lives is surely one of those reasons.